当前位置：首页 > news >正文

MySQL的单表和多表查询

news 2026/2/10 4:38:23

我们在前面曾构建过三个用于实验的表格，下面将基于这三个表进行实践。

# 建立一个用于实验的三个表格
mysql> create table emp (->   empno varchar(10),->   ename varchar(50),->   job varchar(50),->   mgr int,->   hiredate timestamp,->   sal decimal(10, 2),->   comm decimal(10, 2),->   deptno int-> );
Query OK, 0 rows affected (0.03 sec)mysql> insert into emp (empno, ename, job, mgr, hiredate, sal, comm, deptno) values-> ('007369', 'smith', 'clerk', 7902, '1980-12-17 00:00:00', 800.00, null, 20),-> ('007499', 'allen', 'salesman', 7698, '1981-02-20 00:00:00', 1600.00, 300.00, 30),-> ('007521', 'ward', 'salesman', 7698, '1981-02-22 00:00:00', 1250.00, 500.00, 30),-> ('007566', 'jones', 'manager', 7839, '1981-04-02 00:00:00', 2975.00, null, 20),-> ('007654', 'martin', 'salesman', 7698, '1981-09-28 00:00:00', 1250.00, 1400.00, 30),-> ('007698', 'blake', 'manager', 7839, '1981-05-01 00:00:00', 2850.00, null, 30),-> ('007782', 'clark', 'manager', 7839, '1981-06-09 00:00:00', 2450.00, null, 10),-> ('007788', 'scott', 'analyst', 7566, '1987-04-19 00:00:00', 3000.00, null, 20),-> ('007839', 'king', 'president', null, '1981-11-17 00:00:00', 5000.00, null, 10),-> ('007844', 'turner', 'salesman', 7698, '1981-09-08 00:00:00', 1500.00, 0.00, 30),-> ('007876', 'adams', 'clerk', 7788, '1987-05-23 00:00:00', 1100.00, null, 20),-> ('007900', 'james', 'clerk', 7698, '1981-12-03 00:00:00', 950.00, null, 30),-> ('007902', 'ford', 'analyst', 7566, '1981-12-03 00:00:00', 3000.00, null, 20),-> ('007934', 'miller', 'clerk', 7782, '1982-01-23 00:00:00', 1300.00, null, 10);
Query OK, 14 rows affected (0.01 sec)
Records: 14  Duplicates: 0  Warnings: 0mysql> create table dept (->   deptno int,->   dname varchar(50),->   loc varchar(50)-> );
Query OK, 0 rows affected (0.03 sec)mysql> insert into dept (deptno, dname, loc) values-> (10, 'accounting', 'new york'),-> (20, 'research', 'dallas'),-> (30, 'sales', 'chicago'),-> (40, 'operations', 'boston');
Query OK, 4 rows affected (0.01 sec)
Records: 4  Duplicates: 0  Warnings: 0mysql> create table salgrade (->   grade int,->   losal int,->   hisal int-> );
Query OK, 0 rows affected (0.03 sec)mysql> insert into salgrade (grade, losal, hisal) values-> (1, 700, 1200),-> (2, 1201, 1400),-> (3, 1401, 2000),-> (4, 2001, 3000),-> (5, 3001, 9999);
Query OK, 5 rows affected (0.01 sec)
Records: 5  Duplicates: 0  Warnings: 0mysql> select * from emp;
+--------+--------+-----------+------+---------------------+---------+---------+--------+
| empno  | ename  | job       | mgr  | hiredate            | sal     | comm    | deptno |
+--------+--------+-----------+------+---------------------+---------+---------+--------+
| 007369 | smith  | clerk     | 7902 | 1980-12-17 00:00:00 |  800.00 |    NULL |     20 |
| 007499 | allen  | salesman  | 7698 | 1981-02-20 00:00:00 | 1600.00 |  300.00 |     30 |
| 007521 | ward   | salesman  | 7698 | 1981-02-22 00:00:00 | 1250.00 |  500.00 |     30 |
| 007566 | jones  | manager   | 7839 | 1981-04-02 00:00:00 | 2975.00 |    NULL |     20 |
| 007654 | martin | salesman  | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 |     30 |
| 007698 | blake  | manager   | 7839 | 1981-05-01 00:00:00 | 2850.00 |    NULL |     30 |
| 007782 | clark  | manager   | 7839 | 1981-06-09 00:00:00 | 2450.00 |    NULL |     10 |
| 007788 | scott  | analyst   | 7566 | 1987-04-19 00:00:00 | 3000.00 |    NULL |     20 |
| 007839 | king   | president | NULL | 1981-11-17 00:00:00 | 5000.00 |    NULL |     10 |
| 007844 | turner | salesman  | 7698 | 1981-09-08 00:00:00 | 1500.00 |    0.00 |     30 |
| 007876 | adams  | clerk     | 7788 | 1987-05-23 00:00:00 | 1100.00 |    NULL |     20 |
| 007900 | james  | clerk     | 7698 | 1981-12-03 00:00:00 |  950.00 |    NULL |     30 |
| 007902 | ford   | analyst   | 7566 | 1981-12-03 00:00:00 | 3000.00 |    NULL |     20 |
| 007934 | miller | clerk     | 7782 | 1982-01-23 00:00:00 | 1300.00 |    NULL |     10 |
+--------+--------+-----------+------+---------------------+---------+---------+--------+
14 rows in set (0.00 sec)mysql> select * from dept;
+--------+------------+----------+
| deptno | dname      | loc      |
+--------+------------+----------+
|     10 | accounting | new york |
|     20 | research   | dallas   |
|     30 | sales      | chicago  |
|     40 | operations | boston   |
+--------+------------+----------+
4 rows in set (0.00 sec)mysql> select * from salgrade;
+-------+-------+-------+
| grade | losal | hisal |
+-------+-------+-------+
|     1 |   700 |  1200 |
|     2 |  1201 |  1400 |
|     3 |  1401 |  2000 |
|     4 |  2001 |  3000 |
|     5 |  3001 |  9999 |
+-------+-------+-------+
5 rows in set (0.00 sec)

还需要使得表之间有联系。

# 关联表格
# 设置主键
mysql> alter table dept modify column deptno int not null, add primary key (deptno);
Query OK, 0 rows affected (0.07 sec)
Records: 0  Duplicates: 0  Warnings: 0# 添加外键
mysql> alter table emp add foreign key (deptno) references dept(deptno);
Query OK, 14 rows affected (0.08 sec)
Records: 14  Duplicates: 0  Warnings: 0# 查看设置
mysql> desc emp;
+----------+---------------+------+-----+-------------------+-----------------------------+
| Field    | Type          | Null | Key | Default           | Extra                       |
+----------+---------------+------+-----+-------------------+-----------------------------+
| empno    | varchar(10)   | YES  |     | NULL              |                             |
| ename    | varchar(50)   | YES  |     | NULL              |                             |
| job      | varchar(50)   | YES  |     | NULL              |                             |
| mgr      | int(11)       | YES  |     | NULL              |                             |
| hiredate | timestamp     | NO   |     | CURRENT_TIMESTAMP | on update CURRENT_TIMESTAMP |
| sal      | decimal(10,2) | YES  |     | NULL              |                             |
| comm     | decimal(10,2) | YES  |     | NULL              |                             |
| deptno   | int(11)       | YES  | MUL | NULL              |                             |
+----------+---------------+------+-----+-------------------+-----------------------------+
8 rows in set (0.00 sec)mysql> desc dept;
+--------+-------------+------+-----+---------+-------+
| Field  | Type        | Null | Key | Default | Extra |
+--------+-------------+------+-----+---------+-------+
| deptno | int(11)     | NO   | PRI | NULL    |       |
| dname  | varchar(50) | YES  |     | NULL    |       |
| loc    | varchar(50) | YES  |     | NULL    |       |
+--------+-------------+------+-----+---------+-------+
3 rows in set (0.00 sec)

1.单表查询

查询工资高于 1000 或岗位为 manager 的雇员，同时还要满足他们的姓名首字母为 j

# 需求 1
mysql> select * from emp where ((sal>1000 or job='manager') and left(ename, 1)='j');
+--------+-------+---------+------+---------------------+---------+------+--------+
| empno  | ename | job     | mgr  | hiredate            | sal     | comm | deptno |
+--------+-------+---------+------+---------------------+---------+------+--------+
| 007566 | jones | manager | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL |     20 |
+--------+-------+---------+------+---------------------+---------+------+--------+
1 row in set (0.00 sec)select * from EMP where (sal>500 or job='MANAGER') and ename like 'J%';
mysql> select * from emp where (sal>1000 or job='manager') and ename like 'J%';;
+--------+-------+---------+------+---------------------+---------+------+--------+
| empno  | ename | job     | mgr  | hiredate            | sal     | comm | deptno |
+--------+-------+---------+------+---------------------+---------+------+--------+
| 007566 | jones | manager | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL |     20 |
+--------+-------+---------+------+---------------------+---------+------+--------+
1 row in set (0.00 sec)

按照部门号升序、雇员的工资降序来排序

# 需求 2
mysql> select sal, deptno, ename from emp order by deptno asc, sal desc;
+---------+--------+--------+
| sal     | deptno | ename  |
+---------+--------+--------+
| 5000.00 |     10 | king   |
| 2450.00 |     10 | clark  |
| 1300.00 |     10 | miller |
| 3000.00 |     20 | scott  |
| 3000.00 |     20 | ford   |
| 2975.00 |     20 | jones  |
| 1100.00 |     20 | adams  |
|  800.00 |     20 | smith  |
| 2850.00 |     30 | blake  |
| 1600.00 |     30 | allen  |
| 1500.00 |     30 | turner |
| 1250.00 |     30 | ward   |
| 1250.00 |     30 | martin |
|  950.00 |     30 | james  |
+---------+--------+--------+
14 rows in set (0.00 sec)

使用年薪（年薪=月薪*12+奖金）进行降序排序

# 需求 3
mysql> select ename, sal*12+ifnull(comm,0) as 年薪 from emp order by '年薪' desc;
+--------+----------+
| ename  | 年薪     |
+--------+----------+
| smith  |  9600.00 |
| allen  | 19500.00 |
| ward   | 15500.00 |
| jones  | 35700.00 |
| martin | 16400.00 |
| blake  | 34200.00 |
| clark  | 29400.00 |
| scott  | 36000.00 |
| king   | 60000.00 |
| turner | 18000.00 |
| adams  | 13200.00 |
| james  | 11400.00 |
| ford   | 36000.00 |
| miller | 15600.00 |
+--------+----------+
14 rows in set (0.00 sec)

显示工资最高的员工的名字和工作岗位

# 需求 4
mysql> select max(sal) from emp;
+----------+
| max(sal) |
+----------+
|  5000.00 |
+----------+
1 row in set (0.00 sec)mysql> select ename, job from emp where sal=5000;
+-------+-----------+
| ename | job       |
+-------+-----------+
| king  | president |
+-------+-----------+
1 row in set (0.00 sec)mysql> select ename, job from emp where sal=(select max(sal) from emp); # 复合查找，也叫“查找子句”
+-------+-----------+
| ename | job       |
+-------+-----------+
| king  | president |
+-------+-----------+
1 row in set (0.01 sec)

显示工资高于平均工资的员工信息

# 需求 5
mysql> select * from emp where sal > (select avg(sal) from emp);
+--------+-------+-----------+------+---------------------+---------+------+--------+
| empno  | ename | job       | mgr  | hiredate            | sal     | comm | deptno |
+--------+-------+-----------+------+---------------------+---------+------+--------+
| 007566 | jones | manager   | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL |     20 |
| 007698 | blake | manager   | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL |     30 |
| 007782 | clark | manager   | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL |     10 |
| 007788 | scott | analyst   | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL |     20 |
| 007839 | king  | president | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL |     10 |
| 007902 | ford  | analyst   | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL |     20 |
+--------+-------+-----------+------+---------------------+---------+------+--------+
6 rows in set (0.00 sec)

显示每个部门的平均工资和最高工资

# 需求 6
mysql> select deptno, avg(sal), max(sal) from emp group by deptno;
+--------+-------------+----------+
| deptno | avg(sal)    | max(sal) |
+--------+-------------+----------+
|     10 | 2916.666667 |  5000.00 |
|     20 | 2175.000000 |  3000.00 |
|     30 | 1566.666667 |  2850.00 |
+--------+-------------+----------+
3 rows in set (0.00 sec)

显示平均工资低于 2000 的部门号和它的平均工资

# 需求 7
mysql> select deptno, avg(sal) 平均工资 from emp group by deptno having 平均工资<2000;
+--------+--------------+
| deptno | 平均工资     |
+--------+--------------+
|     30 |  1566.666667 |
+--------+--------------+
1 row in set (0.01 sec)

显示每种岗位的雇员总数，平均工资

# 需求 8
mysql> select job, format(avg(sal), 2) 平均工资, count(*) 人数 from emp group by job;
+-----------+--------------+--------+
| job       | 平均工资     | 人数   |
+-----------+--------------+--------+
| analyst   | 3,000.00     |      2 |
| clerk     | 1,037.50     |      4 |
| manager   | 2,758.33     |      3 |
| president | 5,000.00     |      1 |
| salesman  | 1,400.00     |      4 |
+-----------+--------------+--------+
5 rows in set (0.00 sec)

2.多表查询

显示雇员名、雇员工资以及所在部门的名字

# 需求 1
# 将表合外表整合为一个表
mysql> select * from emp, dept where emp.deptno=dept.deptno;
+--------+--------+-----------+------+---------------------+---------+---------+--------+--------+------------+----------+
| empno  | ename  | job       | mgr  | hiredate            | sal     | comm    | deptno | deptno | dname      | loc      |
+--------+--------+-----------+------+---------------------+---------+---------+--------+--------+------------+----------+
| 007782 | clark  | manager   | 7839 | 1981-06-09 00:00:00 | 2450.00 |    NULL |     10 |     10 | accounting | new york |
| 007839 | king   | president | NULL | 1981-11-17 00:00:00 | 5000.00 |    NULL |     10 |     10 | accounting | new york |
| 007934 | miller | clerk     | 7782 | 1982-01-23 00:00:00 | 1300.00 |    NULL |     10 |     10 | accounting | new york |
| 007369 | smith  | clerk     | 7902 | 1980-12-17 00:00:00 |  800.00 |    NULL |     20 |     20 | research   | dallas   |
| 007566 | jones  | manager   | 7839 | 1981-04-02 00:00:00 | 2975.00 |    NULL |     20 |     20 | research   | dallas   |
| 007788 | scott  | analyst   | 7566 | 1987-04-19 00:00:00 | 3000.00 |    NULL |     20 |     20 | research   | dallas   |
| 007876 | adams  | clerk     | 7788 | 1987-05-23 00:00:00 | 1100.00 |    NULL |     20 |     20 | research   | dallas   |
| 007902 | ford   | analyst   | 7566 | 1981-12-03 00:00:00 | 3000.00 |    NULL |     20 |     20 | research   | dallas   |
| 007499 | allen  | salesman  | 7698 | 1981-02-20 00:00:00 | 1600.00 |  300.00 |     30 |     30 | sales      | chicago  |
| 007521 | ward   | salesman  | 7698 | 1981-02-22 00:00:00 | 1250.00 |  500.00 |     30 |     30 | sales      | chicago  |
| 007654 | martin | salesman  | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 |     30 |     30 | sales      | chicago  |
| 007698 | blake  | manager   | 7839 | 1981-05-01 00:00:00 | 2850.00 |    NULL |     30 |     30 | sales      | chicago  |
| 007844 | turner | salesman  | 7698 | 1981-09-08 00:00:00 | 1500.00 |    0.00 |     30 |     30 | sales      | chicago  |
| 007900 | james  | clerk     | 7698 | 1981-12-03 00:00:00 |  950.00 |    NULL |     30 |     30 | sales      | chicago  |
+--------+--------+-----------+------+---------------------+---------+---------+--------+--------+------------+----------+
14 rows in set (0.00 sec)mysql> select emp.ename sal dname from emp, dept where emp.deptno=dept.deptno;
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'dname from emp, dept where emp.deptno=dept.deptno' at line 1
mysql> select emp.ename sal, dname, from emp, dept where emp.deptno=dept.deptno;
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'from emp, dept where emp.deptno=dept.deptno' at line 1
mysql> select emp.ename, sal, dname from emp, dept where emp.deptno=dept.deptno;
+--------+---------+------------+
| ename  | sal     | dname      |
+--------+---------+------------+
| clark  | 2450.00 | accounting |
| king   | 5000.00 | accounting |
| miller | 1300.00 | accounting |
| smith  |  800.00 | research   |
| jones  | 2975.00 | research   |
| scott  | 3000.00 | research   |
| adams  | 1100.00 | research   |
| ford   | 3000.00 | research   |
| allen  | 1600.00 | sales      |
| ward   | 1250.00 | sales      |
| martin | 1250.00 | sales      |
| blake  | 2850.00 | sales      |
| turner | 1500.00 | sales      |
| james  |  950.00 | sales      |
+--------+---------+------------+
14 rows in set (0.00 sec)

显示部门号为 10 的部门名，员工名和工资

# 需求 2
mysql> select dept.dname ,emp.ename, emp.sal from emp, dept where emp.deptno=dept.deptno and emp.deptno=10;
+------------+--------+---------+
| dname      | ename  | sal     |
+------------+--------+---------+
| accounting | clark  | 2450.00 |
| accounting | king   | 5000.00 |
| accounting | miller | 1300.00 |
+------------+--------+---------+
3 rows in set (0.00 sec)

显示各个员工的姓名，工资，及工资级别

# 需求 3
# (1)查看工资等级
mysql> select * from salgrade;
+-------+-------+-------+
| grade | losal | hisal |
+-------+-------+-------+
|     1 |   700 |  1200 |
|     2 |  1201 |  1400 |
|     3 |  1401 |  2000 |
|     4 |  2001 |  3000 |
|     5 |  3001 |  9999 |
+-------+-------+-------+
5 rows in set (0.00 sec)# (2)查看员工信息表
mysql> select * from emp;
+--------+--------+-----------+------+---------------------+---------+---------+--------+
| empno  | ename  | job       | mgr  | hiredate            | sal     | comm    | deptno |
+--------+--------+-----------+------+---------------------+---------+---------+--------+
| 007369 | smith  | clerk     | 7902 | 1980-12-17 00:00:00 |  800.00 |    NULL |     20 |
| 007499 | allen  | salesman  | 7698 | 1981-02-20 00:00:00 | 1600.00 |  300.00 |     30 |
| 007521 | ward   | salesman  | 7698 | 1981-02-22 00:00:00 | 1250.00 |  500.00 |     30 |
| 007566 | jones  | manager   | 7839 | 1981-04-02 00:00:00 | 2975.00 |    NULL |     20 |
| 007654 | martin | salesman  | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 |     30 |
| 007698 | blake  | manager   | 7839 | 1981-05-01 00:00:00 | 2850.00 |    NULL |     30 |
| 007782 | clark  | manager   | 7839 | 1981-06-09 00:00:00 | 2450.00 |    NULL |     10 |
| 007788 | scott  | analyst   | 7566 | 1987-04-19 00:00:00 | 3000.00 |    NULL |     20 |
| 007839 | king   | president | NULL | 1981-11-17 00:00:00 | 5000.00 |    NULL |     10 |
| 007844 | turner | salesman  | 7698 | 1981-09-08 00:00:00 | 1500.00 |    0.00 |     30 |
| 007876 | adams  | clerk     | 7788 | 1987-05-23 00:00:00 | 1100.00 |    NULL |     20 |
| 007900 | james  | clerk     | 7698 | 1981-12-03 00:00:00 |  950.00 |    NULL |     30 |
| 007902 | ford   | analyst   | 7566 | 1981-12-03 00:00:00 | 3000.00 |    NULL |     20 |
| 007934 | miller | clerk     | 7782 | 1982-01-23 00:00:00 | 1300.00 |    NULL |     10 |
+--------+--------+-----------+------+---------------------+---------+---------+--------+
14 rows in set (0.00 sec)# (3)整合两表（求笛卡尔积）
mysql> select * from emp, salgrade;
+--------+--------+-----------+------+---------------------+---------+---------+--------+-------+-------+-------+
| empno  | ename  | job       | mgr  | hiredate            | sal     | comm    | deptno | grade | losal | hisal |
+--------+--------+-----------+------+---------------------+---------+---------+--------+-------+-------+-------+
| 007369 | smith  | clerk     | 7902 | 1980-12-17 00:00:00 |  800.00 |    NULL |     20 |     1 |   700 |  1200 |
| 007369 | smith  | clerk     | 7902 | 1980-12-17 00:00:00 |  800.00 |    NULL |     20 |     2 |  1201 |  1400 |
| 007369 | smith  | clerk     | 7902 | 1980-12-17 00:00:00 |  800.00 |    NULL |     20 |     3 |  1401 |  2000 |
| 007369 | smith  | clerk     | 7902 | 1980-12-17 00:00:00 |  800.00 |    NULL |     20 |     4 |  2001 |  3000 |
| 007369 | smith  | clerk     | 7902 | 1980-12-17 00:00:00 |  800.00 |    NULL |     20 |     5 |  3001 |  9999 |
| 007499 | allen  | salesman  | 7698 | 1981-02-20 00:00:00 | 1600.00 |  300.00 |     30 |     1 |   700 |  1200 |
| 007499 | allen  | salesman  | 7698 | 1981-02-20 00:00:00 | 1600.00 |  300.00 |     30 |     2 |  1201 |  1400 |
| 007499 | allen  | salesman  | 7698 | 1981-02-20 00:00:00 | 1600.00 |  300.00 |     30 |     3 |  1401 |  2000 |
| 007499 | allen  | salesman  | 7698 | 1981-02-20 00:00:00 | 1600.00 |  300.00 |     30 |     4 |  2001 |  3000 |
| 007499 | allen  | salesman  | 7698 | 1981-02-20 00:00:00 | 1600.00 |  300.00 |     30 |     5 |  3001 |  9999 |
| 007521 | ward   | salesman  | 7698 | 1981-02-22 00:00:00 | 1250.00 |  500.00 |     30 |     1 |   700 |  1200 |
| 007521 | ward   | salesman  | 7698 | 1981-02-22 00:00:00 | 1250.00 |  500.00 |     30 |     2 |  1201 |  1400 |
| 007521 | ward   | salesman  | 7698 | 1981-02-22 00:00:00 | 1250.00 |  500.00 |     30 |     3 |  1401 |  2000 |
| 007521 | ward   | salesman  | 7698 | 1981-02-22 00:00:00 | 1250.00 |  500.00 |     30 |     4 |  2001 |  3000 |
| 007521 | ward   | salesman  | 7698 | 1981-02-22 00:00:00 | 1250.00 |  500.00 |     30 |     5 |  3001 |  9999 |
| 007566 | jones  | manager   | 7839 | 1981-04-02 00:00:00 | 2975.00 |    NULL |     20 |     1 |   700 |  1200 |
| 007566 | jones  | manager   | 7839 | 1981-04-02 00:00:00 | 2975.00 |    NULL |     20 |     2 |  1201 |  1400 |
| 007566 | jones  | manager   | 7839 | 1981-04-02 00:00:00 | 2975.00 |    NULL |     20 |     3 |  1401 |  2000 |
| 007566 | jones  | manager   | 7839 | 1981-04-02 00:00:00 | 2975.00 |    NULL |     20 |     4 |  2001 |  3000 |
| 007566 | jones  | manager   | 7839 | 1981-04-02 00:00:00 | 2975.00 |    NULL |     20 |     5 |  3001 |  9999 |
| 007654 | martin | salesman  | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 |     30 |     1 |   700 |  1200 |
| 007654 | martin | salesman  | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 |     30 |     2 |  1201 |  1400 |
| 007654 | martin | salesman  | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 |     30 |     3 |  1401 |  2000 |
| 007654 | martin | salesman  | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 |     30 |     4 |  2001 |  3000 |
| 007654 | martin | salesman  | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 |     30 |     5 |  3001 |  9999 |
| 007698 | blake  | manager   | 7839 | 1981-05-01 00:00:00 | 2850.00 |    NULL |     30 |     1 |   700 |  1200 |
| 007698 | blake  | manager   | 7839 | 1981-05-01 00:00:00 | 2850.00 |    NULL |     30 |     2 |  1201 |  1400 |
| 007698 | blake  | manager   | 7839 | 1981-05-01 00:00:00 | 2850.00 |    NULL |     30 |     3 |  1401 |  2000 |
| 007698 | blake  | manager   | 7839 | 1981-05-01 00:00:00 | 2850.00 |    NULL |     30 |     4 |  2001 |  3000 |
| 007698 | blake  | manager   | 7839 | 1981-05-01 00:00:00 | 2850.00 |    NULL |     30 |     5 |  3001 |  9999 |
| 007782 | clark  | manager   | 7839 | 1981-06-09 00:00:00 | 2450.00 |    NULL |     10 |     1 |   700 |  1200 |
| 007782 | clark  | manager   | 7839 | 1981-06-09 00:00:00 | 2450.00 |    NULL |     10 |     2 |  1201 |  1400 |
| 007782 | clark  | manager   | 7839 | 1981-06-09 00:00:00 | 2450.00 |    NULL |     10 |     3 |  1401 |  2000 |
| 007782 | clark  | manager   | 7839 | 1981-06-09 00:00:00 | 2450.00 |    NULL |     10 |     4 |  2001 |  3000 |
| 007782 | clark  | manager   | 7839 | 1981-06-09 00:00:00 | 2450.00 |    NULL |     10 |     5 |  3001 |  9999 |
| 007788 | scott  | analyst   | 7566 | 1987-04-19 00:00:00 | 3000.00 |    NULL |     20 |     1 |   700 |  1200 |
| 007788 | scott  | analyst   | 7566 | 1987-04-19 00:00:00 | 3000.00 |    NULL |     20 |     2 |  1201 |  1400 |
| 007788 | scott  | analyst   | 7566 | 1987-04-19 00:00:00 | 3000.00 |    NULL |     20 |     3 |  1401 |  2000 |
| 007788 | scott  | analyst   | 7566 | 1987-04-19 00:00:00 | 3000.00 |    NULL |     20 |     4 |  2001 |  3000 |
| 007788 | scott  | analyst   | 7566 | 1987-04-19 00:00:00 | 3000.00 |    NULL |     20 |     5 |  3001 |  9999 |
| 007839 | king   | president | NULL | 1981-11-17 00:00:00 | 5000.00 |    NULL |     10 |     1 |   700 |  1200 |
| 007839 | king   | president | NULL | 1981-11-17 00:00:00 | 5000.00 |    NULL |     10 |     2 |  1201 |  1400 |
| 007839 | king   | president | NULL | 1981-11-17 00:00:00 | 5000.00 |    NULL |     10 |     3 |  1401 |  2000 |
| 007839 | king   | president | NULL | 1981-11-17 00:00:00 | 5000.00 |    NULL |     10 |     4 |  2001 |  3000 |
| 007839 | king   | president | NULL | 1981-11-17 00:00:00 | 5000.00 |    NULL |     10 |     5 |  3001 |  9999 |
| 007844 | turner | salesman  | 7698 | 1981-09-08 00:00:00 | 1500.00 |    0.00 |     30 |     1 |   700 |  1200 |
| 007844 | turner | salesman  | 7698 | 1981-09-08 00:00:00 | 1500.00 |    0.00 |     30 |     2 |  1201 |  1400 |
| 007844 | turner | salesman  | 7698 | 1981-09-08 00:00:00 | 1500.00 |    0.00 |     30 |     3 |  1401 |  2000 |
| 007844 | turner | salesman  | 7698 | 1981-09-08 00:00:00 | 1500.00 |    0.00 |     30 |     4 |  2001 |  3000 |
| 007844 | turner | salesman  | 7698 | 1981-09-08 00:00:00 | 1500.00 |    0.00 |     30 |     5 |  3001 |  9999 |
| 007876 | adams  | clerk     | 7788 | 1987-05-23 00:00:00 | 1100.00 |    NULL |     20 |     1 |   700 |  1200 |
| 007876 | adams  | clerk     | 7788 | 1987-05-23 00:00:00 | 1100.00 |    NULL |     20 |     2 |  1201 |  1400 |
| 007876 | adams  | clerk     | 7788 | 1987-05-23 00:00:00 | 1100.00 |    NULL |     20 |     3 |  1401 |  2000 |
| 007876 | adams  | clerk     | 7788 | 1987-05-23 00:00:00 | 1100.00 |    NULL |     20 |     4 |  2001 |  3000 |
| 007876 | adams  | clerk     | 7788 | 1987-05-23 00:00:00 | 1100.00 |    NULL |     20 |     5 |  3001 |  9999 |
| 007900 | james  | clerk     | 7698 | 1981-12-03 00:00:00 |  950.00 |    NULL |     30 |     1 |   700 |  1200 |
| 007900 | james  | clerk     | 7698 | 1981-12-03 00:00:00 |  950.00 |    NULL |     30 |     2 |  1201 |  1400 |
| 007900 | james  | clerk     | 7698 | 1981-12-03 00:00:00 |  950.00 |    NULL |     30 |     3 |  1401 |  2000 |
| 007900 | james  | clerk     | 7698 | 1981-12-03 00:00:00 |  950.00 |    NULL |     30 |     4 |  2001 |  3000 |
| 007900 | james  | clerk     | 7698 | 1981-12-03 00:00:00 |  950.00 |    NULL |     30 |     5 |  3001 |  9999 |
| 007902 | ford   | analyst   | 7566 | 1981-12-03 00:00:00 | 3000.00 |    NULL |     20 |     1 |   700 |  1200 |
| 007902 | ford   | analyst   | 7566 | 1981-12-03 00:00:00 | 3000.00 |    NULL |     20 |     2 |  1201 |  1400 |
| 007902 | ford   | analyst   | 7566 | 1981-12-03 00:00:00 | 3000.00 |    NULL |     20 |     3 |  1401 |  2000 |
| 007902 | ford   | analyst   | 7566 | 1981-12-03 00:00:00 | 3000.00 |    NULL |     20 |     4 |  2001 |  3000 |
| 007902 | ford   | analyst   | 7566 | 1981-12-03 00:00:00 | 3000.00 |    NULL |     20 |     5 |  3001 |  9999 |
| 007934 | miller | clerk     | 7782 | 1982-01-23 00:00:00 | 1300.00 |    NULL |     10 |     1 |   700 |  1200 |
| 007934 | miller | clerk     | 7782 | 1982-01-23 00:00:00 | 1300.00 |    NULL |     10 |     2 |  1201 |  1400 |
| 007934 | miller | clerk     | 7782 | 1982-01-23 00:00:00 | 1300.00 |    NULL |     10 |     3 |  1401 |  2000 |
| 007934 | miller | clerk     | 7782 | 1982-01-23 00:00:00 | 1300.00 |    NULL |     10 |     4 |  2001 |  3000 |
| 007934 | miller | clerk     | 7782 | 1982-01-23 00:00:00 | 1300.00 |    NULL |     10 |     5 |  3001 |  9999 |
+--------+--------+-----------+------+---------------------+---------+---------+--------+-------+-------+-------+
70 rows in set (0.00 sec)# (4)列出员工姓名、员工工资、工资对应等级
mysql> select emp.ename, emp.sal, salgrade.grade from emp, salgrade where sal between salgrade.losal and salgrade.hisal;
+--------+---------+-------+
| ename  | sal     | grade |
+--------+---------+-------+
| smith  |  800.00 |     1 |
| allen  | 1600.00 |     3 |
| ward   | 1250.00 |     2 |
| jones  | 2975.00 |     4 |
| martin | 1250.00 |     2 |
| blake  | 2850.00 |     4 |
| clark  | 2450.00 |     4 |
| scott  | 3000.00 |     4 |
| king   | 5000.00 |     5 |
| turner | 1500.00 |     3 |
| adams  | 1100.00 |     1 |
| james  |  950.00 |     1 |
| ford   | 3000.00 |     4 |
| miller | 1300.00 |     2 |
+--------+---------+-------+
14 rows in set (0.00 sec)

补充：同一张表也可以进行笛卡尔积，也就是“自连接”

# 尝试自连接
mysql> select * from salgrade as t1, salgrade as t2;
+-------+-------+-------+-------+-------+-------+
| grade | losal | hisal | grade | losal | hisal |
+-------+-------+-------+-------+-------+-------+
|     1 |   700 |  1200 |     1 |   700 |  1200 |
|     2 |  1201 |  1400 |     1 |   700 |  1200 |
|     3 |  1401 |  2000 |     1 |   700 |  1200 |
|     4 |  2001 |  3000 |     1 |   700 |  1200 |
|     5 |  3001 |  9999 |     1 |   700 |  1200 |
|     1 |   700 |  1200 |     2 |  1201 |  1400 |
|     2 |  1201 |  1400 |     2 |  1201 |  1400 |
|     3 |  1401 |  2000 |     2 |  1201 |  1400 |
|     4 |  2001 |  3000 |     2 |  1201 |  1400 |
|     5 |  3001 |  9999 |     2 |  1201 |  1400 |
|     1 |   700 |  1200 |     3 |  1401 |  2000 |
|     2 |  1201 |  1400 |     3 |  1401 |  2000 |
|     3 |  1401 |  2000 |     3 |  1401 |  2000 |
|     4 |  2001 |  3000 |     3 |  1401 |  2000 |
|     5 |  3001 |  9999 |     3 |  1401 |  2000 |
|     1 |   700 |  1200 |     4 |  2001 |  3000 |
|     2 |  1201 |  1400 |     4 |  2001 |  3000 |
|     3 |  1401 |  2000 |     4 |  2001 |  3000 |
|     4 |  2001 |  3000 |     4 |  2001 |  3000 |
|     5 |  3001 |  9999 |     4 |  2001 |  3000 |
|     1 |   700 |  1200 |     5 |  3001 |  9999 |
|     2 |  1201 |  1400 |     5 |  3001 |  9999 |
|     3 |  1401 |  2000 |     5 |  3001 |  9999 |
|     4 |  2001 |  3000 |     5 |  3001 |  9999 |
|     5 |  3001 |  9999 |     5 |  3001 |  9999 |
+-------+-------+-------+-------+-------+-------+
25 rows in set (0.00 sec)mysql> select * from salgrade;
+-------+-------+-------+
| grade | losal | hisal |
+-------+-------+-------+
|     1 |   700 |  1200 |
|     2 |  1201 |  1400 |
|     3 |  1401 |  2000 |
|     4 |  2001 |  3000 |
|     5 |  3001 |  9999 |
+-------+-------+-------+
5 rows in set (0.00 sec)

有些情况下是需要自连接的，例如“显示员工 ford 的上级领导的编号和姓名”：

# 尝试寻找上级领导
# (1)查看员工表
mysql> select * from emp;
+--------+--------+-----------+------+---------------------+---------+---------+--------+
| empno  | ename  | job       | mgr  | hiredate            | sal     | comm    | deptno |
+--------+--------+-----------+------+---------------------+---------+---------+--------+
| 007369 | smith  | clerk     | 7902 | 1980-12-17 00:00:00 |  800.00 |    NULL |     20 |
| 007499 | allen  | salesman  | 7698 | 1981-02-20 00:00:00 | 1600.00 |  300.00 |     30 |
| 007521 | ward   | salesman  | 7698 | 1981-02-22 00:00:00 | 1250.00 |  500.00 |     30 |
| 007566 | jones  | manager   | 7839 | 1981-04-02 00:00:00 | 2975.00 |    NULL |     20 |
| 007654 | martin | salesman  | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 |     30 |
| 007698 | blake  | manager   | 7839 | 1981-05-01 00:00:00 | 2850.00 |    NULL |     30 |
| 007782 | clark  | manager   | 7839 | 1981-06-09 00:00:00 | 2450.00 |    NULL |     10 |
| 007788 | scott  | analyst   | 7566 | 1987-04-19 00:00:00 | 3000.00 |    NULL |     20 |
| 007839 | king   | president | NULL | 1981-11-17 00:00:00 | 5000.00 |    NULL |     10 |
| 007844 | turner | salesman  | 7698 | 1981-09-08 00:00:00 | 1500.00 |    0.00 |     30 |
| 007876 | adams  | clerk     | 7788 | 1987-05-23 00:00:00 | 1100.00 |    NULL |     20 |
| 007900 | james  | clerk     | 7698 | 1981-12-03 00:00:00 |  950.00 |    NULL |     30 |
| 007902 | ford   | analyst   | 7566 | 1981-12-03 00:00:00 | 3000.00 |    NULL |     20 |
| 007934 | miller | clerk     | 7782 | 1982-01-23 00:00:00 | 1300.00 |    NULL |     10 |
+--------+--------+-----------+------+---------------------+---------+---------+--------+
14 rows in set (0.00 sec)# (2)查看某个员工的上级领导编号
mysql> select mgr from emp where ename='ford';
+------+
| mgr  |
+------+
| 7566 |
+------+
1 row in set (0.00 sec)# (3)查看领导编号对应的领导姓名
mysql> select ename, empno from emp where empno=7566;
+-------+--------+
| ename | empno  |
+-------+--------+
| jones | 007566 |
+-------+--------+
1 row in set (0.00 sec)# (4)子查询做法
mysql> select ename, empno from emp where empno=(select mgr from emp where ename='ford');
+-------+--------+
| ename | empno  |
+-------+--------+
| jones | 007566 |
+-------+--------+
1 row in set (0.01 sec)# (5)自连接做法
mysql> select e2.empno 领导编号, e2.ename 领导名  from emp as e1, emp as e2 where e1.ename='ford' and e1.mgr=e2.empno;
+--------------+-----------+
| 领导编号     | 领导名    |
+--------------+-----------+
| 007566       | jones     |
+--------------+-----------+
1 row in set (0.00 sec)

MySQL的单表和多表查询

我们在前面曾构建过三个用于实验的表格，下面将基于这三个表进行实践。 # 建立一个用于实验的三个表格 mysql> create table emp (-> empno varchar(10),-> ename varchar(50),-> job varchar(50),-> mgr int,-> hiredate timestamp,-&…...

编程日记 2024/3/1 23:16:42

攻防世界例题wp

1.看到_wakeup()函数第一反应要么触发，要么绕过在这里绕过 2.构造payload实例化一个对象后反序列化 3构造脚本如下： 4.因为它是一个绕过的方法所以我们要使用绕过的方法。 5.继续构造payload将上图的1换成2进行绕过最终的payload为 O:4:"xctf…...

编程日记 2024/3/1 23:14:40

仿牛客网项目---显示评论和添加评论功能的实现

这篇文章，我来介绍一下我的项目中的另外一个功能：显示评论和添加评论。其实这两个功能都不怎么重要，我感觉最重要的应该是用户注册登录功能，这个也了解一下，知道这么一回事儿就好。首先设计DAO层。 Mapper public …...

编程日记 2024/3/1 23:13:39

idea集成git详解教程（实用篇）

0.Git常用命令 Git常用命令-CSDN博客 1.下载git Git - Downloads 一路傻瓜式安装即可（NEXT） 2.软件测试在Windows桌面空白处，点击鼠标右键，弹出右键菜单 Git软件安装后，会在右键菜单中增加两个菜单 Git GUI He…...

编程日记 2024/3/1 23:12:38

搭建LNMP环境并配置个人博客系统

LNMP是Linux（操作系统）、Nginx（Web服务器）、MySQL（数据库）和PHP（脚本解释器）的组合，常用于部署高性能的动态网站，如WordPress等博客平台一、安装Linux操作系…...

编程日记 2024/3/1 23:09:35

为什么网络安全缺口这么大，还是这么缺网络安全工程师？

为啥网安领域缺口多达300多万人，但网安工程师（白帽黑客）却很少，难道又是砖家在忽悠人？？？ 主要原因为这三点: 首先是学校的原因，很多学校网络安全课程用的还都是十年前的老教材&…...

编程日记 2024/3/1 23:06:32

创新洞察｜诺奖获得者埃德蒙·费尔普斯谈未来：中国创新从何而来？

当下，全球宏观经济形势正在发生深刻变化，以创新推进高质量发展被推向前所未有的高度。作为多年与中国经济结伴而行的经济学家、2006年诺贝尔经济学奖得主，埃德蒙费尔普斯认为：创新无处不在，而中国人具有巨大的创新活力…...

编程日记 2024/3/1 23:03:24

SpringMVC 学习（六）之视图

目录 1 SpringMVC 视图介绍 2 JSP 视图 3 Thymeleaf 视图 4 FreeMarker 视图 5 XSLT 视图 6 请求转发与重定向 6.1 请求转发 (Forward) 6.2 重定向 (Redirect) 7 视图控制器 (view-controller) 1 SpringMVC 视图介绍在 SpringMVC 框架中，视图可以是一个 J…...

编程日记 2024/3/1 23:02:23

ROS 2基础概念#1：计算图（Compute Graph）| ROS 2学习笔记

在ROS中，计算图（ROS Compute Graph）是一个核心概念，它描述了ROS节点之间的数据流动和通信方式。它不仅仅是一个通信网络，它也反映了ROS设计哲学的核心——灵活性、模块化和可重用性。通过细致探讨计算图的高级特性和实…...

编程日记 2024/3/1 23:00:21

一本通 1403：素数对

在判断素数对的两个数是否都为素数时可以只判断数的一半 #include<bits/stdc.h> using namespace std; bool su(int a,int b){ for(int i2;i<sqrt(a);i){ if(a%i0){ return 0; } } for(int i2;i<sqrt(b);i){ if(…...

编程日记 2024/3/1 22:56:18

华为---RSTP（四）---RSTP的保护功能简介和示例配置

目录 1. 技术背景 2. RSTP的保护功能 3. BPDU保护机制原理和配置命令 3.1 BPDU保护机制原理 3.2 BPDU保护机制配置命令 3.3 BPDU保护机制配置步骤 4. 根保护机制原理和配置命令 4.1 根保护机制原理 4.2 根保护机制配置命令 4.3 根保护机制配置步骤 5. 环路保护机…...

编程日记 2024/3/1 22:52:14

Android基础控件介绍

在Android应用程序开发中，使用基础控件是非常常见的。这些控件允许您在用户界面中显示文本、图像、按钮等元素，以及接收用户输入。本文将介绍几种常见的基础控件，并给出每个控件在示例XML中使用的属性的详细说明。 1. TextView TextView 是…...

编程日记 2024/3/1 22:48:11

【总结】Maxwell学习笔记

1.Maxwell简介 Maxwell 是一款用Java编写的MySQL变更数据抓取软件，它会实时监控Mysql数据库的数据变更操作（包括insert、update、delete），并将变更数据以 JSON 格式发送给 Kafka、Kinesi等流数据处理平台官网地址：M…...

编程日记 2024/3/1 22:46:07

AFL fork server和fuzzer的交互

看了一些博客，都是在说fuzzer和fork server进行交互，由fork server fork出子进程来执行程序，但是不太明白这两者到底是如何在代码层面进行交互的。 run_target中有这么一段代码，大概意思是fuzzer给fork server传递prev_timed_out…...

编程日记 2024/3/1 22:43:04

Java SE：多线程（Thread）

1. 线程两个基本概念并发：即线程交替运行多个指令并行：即多个线程同时运行指令并发并行不矛盾，两者可同时发生，即多个线程交替运行指令 2. 多线程3种实现方式 2.1 直接创建线程对象 /*** 方式1：* 1. 创建thread类的…...

编程日记 2024/3/1 22:42:03

你敢信？软件测试万能面试脚本他来了？

写在前面： 过春节了，四舍五入接下来马上要年底了，新一波的跳槽旺季马上来临，不知道你是不是已经安于现状，还是蓄势待发呢？最近我和我的哈皮群友们一顿讨论，拟写了大家可能会遇到的面试情况&…...

编程日记 2024/3/1 22:41:02

C++/Python简单练手题

前言最近需要开始使用python，但是对python了解的并不多，于是先从很早之前刚学C时写过的一些练手题开始，使用python来实现相同的功能，在温习python基础语法的同时，也一起来感受感受python的魅力 99乘法表 c&#xf…...

编程日记 2024/3/1 22:40:00

视频在线压缩

video2edit 一款免费的在线视频编辑软件，可以进行视频合并、视频剪辑、视频压缩以及转换视频格式等。链接地址：在线视频编辑器和转换器 - 编辑，转换和压缩视频文件打开视频压缩页面，上传想要压缩视频，支持MP4&…...

编程日记 2024/3/1 22:38:59

Python列表的合并、重复、判断与切片操作你学会了吗

1.合并列表通过实现 list1 ["佛跳墙", "肠粉", "刀削面", "烤鸭"]list2 [32, 4, 5, 7.43, True]list3 list1 list2print(list3) # [佛跳墙, 肠粉, 刀削面, 烤鸭, 32, 4, 5, 7.43, True] 2.重复输出列表中的元素通过 * 实…...

编程日记 2024/3/1 22:37:58

Vue(3.3.4)+three.js(0.161.0)实现3D可视化地图

一.前言由于最近在学习three.js,所以观摩了一下掘金，csdn等网站上的有关这部分的内容，刚好看到一个带你入门three.js——从0到1实现一个3d可视化地图 - 掘金 (juejin.cn)，再加上我的专业属性是地理相关，可以说是专业对口&#xf…...

编程日记 2024/3/1 22:35:56

【大模型RAG】拍照搜题技术架构速览：三层管道、两级检索、兜底大模型

摘要拍照搜题系统采用“三层管道（多模态 OCR → 语义检索 → 答案渲染）、两级检索（倒排 BM25 向量 HNSW）并以大语言模型兜底”的整体框架： 多模态 OCR 层将题目图片经过超分、去噪、倾斜校正后，分别用…...

编程新知 2026/2/8 4:32:38

测试微信模版消息推送

进入“开发接口管理”--“公众平台测试账号”，无需申请公众账号、可在测试账号中体验并测试微信公众平台所有高级接口。获取access_token: 自定义模版消息： 关注测试号：扫二维码关注测试号。发送模版消息： import requests da…...

编程新知 2026/2/8 4:37:13

k8s从入门到放弃之Ingress七层负载

k8s从入门到放弃之Ingress七层负载在Kubernetes（简称K8s）中，Ingress是一个API对象，它允许你定义如何从集群外部访问集群内部的服务。Ingress可以提供负载均衡、SSL终结和基于名称的虚拟主机等功能。通过Ingress，你可…...

编程新知 2025/9/7 0:12:53

深入浅出：JavaScript 中的 `window.crypto.getRandomValues()` 方法

深入浅出：JavaScript 中的 window.crypto.getRandomValues() 方法在现代 Web 开发中，随机数的生成看似简单，却隐藏着许多玄机。无论是生成密码、加密密钥，还是创建安全令牌，随机数的质量直接关系到系统的安全性。Jav…...

编程新知 2025/10/8 4:56:09

1688商品列表API与其他数据源的对接思路

将1688商品列表API与其他数据源对接时，需结合业务场景设计数据流转链路，重点关注数据格式兼容性、接口调用频率控制及数据一致性维护。以下是具体对接思路及关键技术点： 一、核心对接场景与目标商品数据同步场景：将1688商品信息…...

编程新知 2025/11/30 16:55:32

多模态商品数据接口：融合图像、语音与文字的下一代商品详情体验

一、多模态商品数据接口的技术架构 （一）多模态数据融合引擎跨模态语义对齐通过Transformer架构实现图像、语音、文字的语义关联。例如，当用户上传一张“蓝色连衣裙”的图片时，接口可自动提取图像中的颜色（RGB值&…...

编程新知 2025/7/23 3:55:49

dify打造数据可视化图表

一、概述在日常工作和学习中，我们经常需要和数据打交道。无论是分析报告、项目展示，还是简单的数据洞察，一个清晰直观的图表，往往能胜过千言万语。一款能让数据可视化变得超级简单的 MCP Server，由蚂蚁集团 AntV 团队…...

编程新知 2026/1/20 20:10:03

关键领域软件测试的突围之路：如何破解安全与效率的平衡难题

在数字化浪潮席卷全球的今天，软件系统已成为国家关键领域的核心战斗力。不同于普通商业软件，这些承载着国家安全使命的软件系统面临着前所未有的质量挑战——如何在确保绝对安全的前提下，实现高效测试与快速迭代？这一命题正考验着…...

编程新知 2026/1/27 13:20:11

Python 包管理器 uv 介绍

Python 包管理器 uv 全面介绍 uv 是由 Astral（热门工具 Ruff 的开发者）推出的下一代高性能 Python 包管理器和构建工具，用 Rust 编写。它旨在解决传统工具（如 pip、virtualenv、pip-tools）的性能瓶颈，同时…...

编程新知 2026/2/7 10:47:09

高效线程安全的单例模式：Python 中的懒加载与自定义初始化参数

高效线程安全的单例模式：Python 中的懒加载与自定义初始化参数在软件开发中，单例模式（Singleton Pattern）是一种常见的设计模式，确保一个类仅有一个实例，并提供一个全局访问点。在多线程环境下，实现单例模式时需要注意线程安全问题，以防止多个线程同时创建实例，导致…...

编程新知 2025/11/25 19:50:27

1.单表查询

2.多表查询

相关文章：