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MySQL 基本查询

news 2025/7/27 7:03:36

表的增删改查

CRUD 是数据库操作的四种基本类型，分别对应：增删查改

  • Create（创建）

  • Retrieve（读取）

  • Update（更新）

  • Delete（删除）

1. Create（创建）
1.1 单行数据 + 全列插入

语法：

INSERT INTO table_name [(column [, column] ...)] VALUES (value_list);

案例：

-- 创建一张学生表
CREATE TABLE students (id INT UNSIGNED PRIMARY KEY AUTO_INCREMENT,sn INT NOT NULL UNIQUE COMMENT '学号',name VARCHAR(20) NOT NULL,qq VARCHAR(20)
);-- 插入单行数据
INSERT INTO students VALUES (101, 10001, '孙悟空', '11111');
Query OK, 1 row affected (0.02 sec)-- 插入两条记录，value_list 数量必须和定义表的列的数量及顺序一致
INSERT INTO students VALUES (100, 10000, '唐三藏', NULL);
Query OK, 1 row affected (0.02 sec)-- 查看插入结果
SELECT * FROM students;
+-----+-------+-----------+-------+
| id  | sn    | name      | qq    |
+-----+-------+-----------+-------+
| 100 | 10000 | 唐三藏    | NULL  |
| 101 | 10001 | 孙悟空    | 11111 |
+-----+-------+-----------+-------+
2 rows in set (0.00 sec)
1.2 多行数据 + 指定列插入

语法：

INSERT INTO table_name (column [, column] ...) VALUES (value_list) [, (value_list)] ...;

案例：

-- 插入多行数据
INSERT INTO students (id, sn, name) VALUES
(102, 20001, '曹孟德'),
(103, 20002, '孙仲谋');
Query OK, 2 rows affected (0.02 sec)
Records: 2  Duplicates: 0  Warnings: 0-- 查看插入结果
SELECT * FROM students;
+-----+-------+-----------+-------+
| id  | sn    | name      | qq    |
+-----+-------+-----------+-------+
| 100 | 10000 | 唐三藏    | NULL  |
| 101 | 10001 | 孙悟空    | 11111 |
| 102 | 20001 | 曹孟德    | NULL  |
| 103 | 20002 | 孙仲谋    | NULL  |
+-----+-------+-----------+-------+
4 rows in set (0.00 sec)
1.3 插入否则更新

当主键或唯一键对应的值已经存在时，可以选择性地进行更新操作。

语法：

INSERT INTO table_name (column [, column] ...) VALUES (value_list)
ON DUPLICATE KEY UPDATE column = value [, column = value] ...;

案例：

-- 主键冲突
INSERT INTO students (id, sn, name) VALUES (100, 10010, '唐大师');
ERROR 1062 (23000): Duplicate entry '100' for key 'PRIMARY'-- 唯一键冲突
INSERT INTO students (sn, name) VALUES (20001, '曹阿瞒');
ERROR 1062 (23000): Duplicate entry '20001' for key 'sn'-- 插入否则更新
INSERT INTO students (id, sn, name) VALUES (100, 10010, '唐大师')
ON DUPLICATE KEY UPDATE sn = 10010, name = '唐大师';
Query OK, 2 rows affected (0.47 sec)-- 获取受影响的行数
SELECT ROW_COUNT();
+-------------+
| ROW_COUNT() |
+-------------+
| 2           |
+-------------+
1.4 替换

语法：

REPLACE INTO table_name (column [, column] ...) VALUES (value_list) [, (value_list)] ...;

案例：

-- 替换数据
REPLACE INTO students (sn, name) VALUES (20001, '曹阿瞒');
Query OK, 2 rows affected (0.00 sec)
2. Retrieve（读取）
2.1 SELECT 列

语法：

SELECT [DISTINCT] {column [, column] ... | *} FROM table_name
[WHERE ...]
[ORDER BY column [ASC | DESC], ...]
[LIMIT ...];

案例：

-- 创建表结构
CREATE TABLE exam_result (id INT UNSIGNED PRIMARY KEY AUTO_INCREMENT,name VARCHAR(20) NOT NULL COMMENT '同学姓名',chinese FLOAT DEFAULT 0.0 COMMENT '语文成绩',math FLOAT DEFAULT 0.0 COMMENT '数学成绩',english FLOAT DEFAULT 0.0 COMMENT '英语成绩'
);-- 插入测试数据
INSERT INTO exam_result (name, chinese, math, english) VALUES
('唐三藏', 67, 98, 56),
('孙悟空', 87, 78, 77),
('猪悟能', 88, 98, 90),
('曹孟德', 82, 84, 67),
('刘玄德', 55, 85, 45),
('孙权', 70, 73, 78),
('宋公明', 75, 65, 30);
Query OK, 7 rows affected (0.00 sec)
Records: 7  Duplicates: 0  Warnings: 0-- 查询所有列
SELECT * FROM exam_result;
+----+-----------+-------+--------+--------+
| id | name      | chinese | math  | english |
+----+-----------+-------+--------+--------+
| 1  | 唐三藏    | 67    | 98     | 56     |
| 2  | 孙悟空    | 87    | 78     | 77     |
| 3  | 猪悟能    | 88    | 98     | 90     |
| 4  | 曹孟德    | 82    | 84     | 67     |
| 5  | 刘玄德    | 55    | 85     | 45     |
| 6  | 孙权      | 70    | 73     | 78     |
| 7  | 宋公明    | 75    | 65     | 30     |
+----+-----------+-------+--------+--------+
7 rows in set (0.00 sec)
2.2 全列查询

通常不建议使用 * 进行全列查询，因为：

  1. 查询的列越多，传输的数据量越大。

  2. 可能会影响索引的使用。

2.3 指定列查询
-- 指定列查询
SELECT id, name, english FROM exam_result;
+----+-----------+--------+
| id | name      | english |
+----+-----------+--------+
| 1  | 唐三藏    | 56     |
| 2  | 孙悟空    | 77     |
| 3  | 猪悟能    | 90     |
| 4  | 曹孟德    | 67     |
| 5  | 刘玄德    | 45     |
| 6  | 孙权      | 78     |
| 7  | 宋公明    | 30     |
+----+-----------+--------+
7 rows in set (0.00 sec)
2.4 查询字段为表达式
-- 表达式不包含字段
SELECT id, name, 10 FROM exam_result;
+----+-----------+----+
| id | name      | 10 |
+----+-----------+----+
| 1  | 唐三藏    | 10 |
| 2  | 孙悟空    | 10 |
| 3  | 猪悟能    | 10 |
| 4  | 曹孟德    | 10 |
| 5  | 刘玄德    | 10 |
| 6  | 孙权      | 10 |
| 7  | 宋公明    | 10 |
+----+-----------+----+
7 rows in set (0.00 sec)-- 表达式包含一个字段
SELECT id, name, english + 10 FROM exam_result;
+----+-----------+-------------+
| id | name      | english + 10 |
+----+-----------+-------------+
| 1  | 唐三藏    | 66          |
| 2  | 孙悟空    | 87          |
| 3  | 猪悟能    | 100         |
| 4  | 曹孟德    | 77          |
| 5  | 刘玄德    | 55          |
| 6  | 孙权      | 88          |
| 7  | 宋公明    | 40          |
+----+-----------+-------------+
7 rows in set (0.00 sec)-- 表达式包含多个字段
SELECT id, name, chinese + math + english FROM exam_result;
+----+-----------+-------------------------+
| id | name      | chinese + math + english |
+----+-----------+-------------------------+
| 1  | 唐三藏    | 221                     |
| 2  | 孙悟空    | 242                     |
| 3  | 猪悟能    | 276                     |
| 4  | 曹孟德    | 233                     |
| 5  | 刘玄德    | 185                     |
| 6  | 孙权      | 221                     |
| 7  | 宋公明    | 170                     |
+----+-----------+-------------------------+
7 rows in set (0.00 sec)
2.5 为查询结果指定别名

语法：

SELECT column [AS] alias_name [...] FROM table_name;

案例：

-- 为查询结果指定别名
SELECT id, name, chinese + math + english AS 总分 FROM exam_result;
+----+-----------+--------+
| id | name      | 总分   |
+----+-----------+--------+
| 1  | 唐三藏    | 221    |
| 2  | 孙悟空    | 242    |
| 3  | 猪悟能    | 276    |
| 4  | 曹孟德    | 233    |
| 5  | 刘玄德    | 185    |
| 6  | 孙权      | 221    |
| 7  | 宋公明    | 170    |
+----+-----------+--------+
7 rows in set (0.00 sec)
2.6 结果去重
-- 查询数学成绩
SELECT math FROM exam_result;
+--------+
| math   |
+--------+
| 98     |
| 78     |
| 98     |
| 84     |
| 85     |
| 73     |
| 65     |
+--------+
7 rows in set (0.00 sec)-- 去重结果
SELECT DISTINCT math FROM exam_result;
+--------+
| math   |
+--------+
| 98     |
| 78     |
| 84     |
| 85     |
| 73     |
| 65     |
+--------+
6 rows in set (0.00 sec)
2.7 WHERE 条件

比较运算符：

运算符说明
>, >=, <, <=大于、大于等于、小于、小于等于
=等于，NULL 不安全，例如 NULL = NULL 的结果是 NULL
<=>等于，NULL 安全，例如 NULL <=> NULL 的结果是 TRUE(1)
!=, <>不等于
BETWEEN a0 AND a1范围匹配，[a0, a1]，如果 a0 <= value <= a1，返回 TRUE(1)
IN (option, ...)如果是 option 中的任意一个，返回 TRUE(1)
IS NULL是 NULL
IS NOT NULL不是 NULL
LIKE模糊匹配。% 表示任意多个（包括 0 个）任意字符；_ 表示任意一个字符

逻辑运算符：

运算符说明
AND多个条件必须都为 TRUE(1)，结果才是 TRUE(1)
OR任意一个条件为 TRUE(1)，结果为 TRUE(1)
NOT条件为 TRUE(1)，结果为 FALSE(0)

案例：

-- 英语不及格的同学及英语成绩 (< 60)
SELECT name, english FROM exam_result WHERE english < 60;
+-----------+--------+
| name      | english |
+-----------+--------+
| 唐三藏    | 56     |
| 刘玄德    | 45     |
| 宋公明    | 30     |
+-----------+--------+
3 rows in set (0.01 sec)-- 语文成绩在 [80, 90] 分的同学及语文成绩
SELECT name, chinese FROM exam_result WHERE chinese >= 80 AND chinese <= 90;
+-----------+-------+
| name      | chinese |
+-----------+-------+
| 孙悟空    | 87     |
| 猪悟能    | 88     |
| 曹孟德    | 82     |
+-----------+-------+
3 rows in set (0.00 sec)-- 使用 BETWEEN ... AND ... 条件
SELECT name, chinese FROM exam_result WHERE chinese BETWEEN 80 AND 90;
+-----------+-------+
| name      | chinese |
+-----------+-------+
| 孙悟空    | 87     |
| 猪悟能    | 88     |
| 曹孟德    | 82     |
+-----------+-------+
3 rows in set (0.00 sec)-- 数学成绩是 58 或者 59 或者 98 或者 99 分的同学及数学成绩
SELECT name, math FROM exam_result
WHERE math = 58OR math = 59OR math = 98OR math = 99;
+-----------+--------+
| name      | math   |
+-----------+--------+
| 唐三藏    | 98     |
| 猪悟能    | 98     |
+-----------+--------+
2 rows in set (0.01 sec)-- 使用 IN 条件
SELECT name, math FROM exam_result WHERE math IN (58, 59, 98, 99);
+-----------+--------+
| name      | math   |
+-----------+--------+
| 唐三藏    | 98     |
| 猪悟能    | 98     |
+-----------+--------+
2 rows in set (0.00 sec)-- 姓孙的同学及孙某同学
SELECT name FROM exam_result WHERE name LIKE '孙%';
+-----------+
| name      |
+-----------+
| 孙悟空    |
| 孙权      |
+-----------+
2 rows in set (0.00 sec)-- 匹配严格的一个任意字符
SELECT name FROM exam_result WHERE name LIKE '孙_';
+--------+
| name   |
+--------+
| 孙权   |
+--------+
1 row in set (0.00 sec)-- 语文成绩好于英语成绩的同学
SELECT name, chinese, english FROM exam_result WHERE chinese > english;
+-----------+-------+--------+
| name      | chinese | english |
+-----------+-------+--------+
| 唐三藏    | 67     | 56     |
| 孙悟空    | 87     | 77     |
| 曹孟德    | 82     | 67     |
| 刘玄德    | 55     | 45     |
| 宋公明    | 75     | 30     |
+-----------+-------+--------+
5 rows in set (0.00 sec)-- 总分在 200 分以下的同学
SELECT name, chinese + math + english AS 总分 FROM exam_result
WHERE chinese + math + english < 200;
+-----------+--------+
| name      | 总分   |
+-----------+--------+
| 刘玄德    | 185    |
| 宋公明    | 170    |
+-----------+--------+
2 rows in set (0.00 sec)-- 语文成绩 > 80 并且不姓孙的同学
SELECT name, chinese FROM exam_result
WHERE chinese > 80 AND name NOT LIKE '孙%';
+-----------+-------+
| name      | chinese |
+-----------+-------+
| 猪悟能    | 88     |
| 曹孟德    | 82     |
+-----------+-------+
2 rows in set (0.00 sec)-- 孙某同学，否则要求总成绩 > 200 并且 语文成绩 < 数学成绩 并且 英语成绩 > 80
SELECT name, chinese, math, english, chinese + math + english AS 总分
FROM exam_result
WHERE name LIKE '孙_' OR (chinese + math + english > 200 AND chinese < math AND english > 80
);
+-----------+-------+--------+--------+--------+
| name      | chinese | math  | english | 总分   |
+-----------+-------+--------+--------+--------+
| 猪悟能    | 88     | 98     | 90     | 276    |
| 孙权      | 70     | 73     | 78     | 221    |
+-----------+-------+--------+--------+--------+
2 rows in set (0.00 sec)-- 查询 qq 号已知的同学姓名
SELECT name, qq FROM students WHERE qq IS NOT NULL;
+-----------+-------+
| name      | qq    |
+-----------+-------+
| 孙悟空    | 11111 |
+-----------+-------+
1 row in set (0.00 sec)-- NULL 和 NULL 的比较，= 和 <=> 的区别
SELECT NULL = NULL, NULL = 1, NULL = 0;
+-------------+----------+----------+
| NULL = NULL | NULL = 1 | NULL = 0 |
+-------------+----------+----------+
| NULL        | NULL     | NULL     |
+-------------+----------+----------+
1 row in set (0.00 sec)SELECT NULL <=> NULL, NULL <=> 1, NULL <=> 0;
+---------------+------------+------------+
| NULL <=> NULL | NULL <=> 1 | NULL <=> 0 |
+---------------+------------+------------+
| 1             | 0          | 0          |
+---------------+------------+------------+
1 row in set (0.00 sec)
2.8 结果排序

语法：

SELECT ... FROM table_name [WHERE ...]
ORDER BY column [ASC|DESC], [...];

案例：

-- 同学及数学成绩，按数学成绩升序显示
SELECT name, math FROM exam_result ORDER BY math;
+-----------+--------+
| name      | math   |
+-----------+--------+
| 宋公明    | 65     |
| 孙权      | 73     |
| 孙悟空    | 78     |
| 曹孟德    | 84     |
| 刘玄德    | 85     |
| 唐三藏    | 98     |
| 猪悟能    | 98     |
+-----------+--------+
7 rows in set (0.00 sec)-- 同学及 qq 号，按 qq 号排序显示
SELECT name, qq FROM students ORDER BY qq;
+-----------+-------+
| name      | qq    |
+-----------+-------+
| 唐大师    | NULL  |
| 孙仲谋    | NULL  |
| 曹阿瞒    | NULL  |
| 孙悟空    | 11111 |
+-----------+-------+
4 rows in set (0.00 sec)-- NULL 视为比任何值都小，降序出现在最下面
SELECT name, qq FROM students ORDER BY qq DESC;
+-----------+-------+
| name      | qq    |
+-----------+-------+
| 孙悟空    | 11111 |
| 唐大师    | NULL  |
| 孙仲谋    | NULL  |
| 曹阿瞒    | NULL  |
+-----------+-------+
4 rows in set (0.00 sec)-- 查询同学各门成绩，依次按 数学降序，英语升序，语文升序的方式显示
SELECT name, math, english, chinese FROM exam_result
ORDER BY math DESC, english, chinese;
+-----------+--------+--------+-------+
| name      | math   | english | chinese |
+-----------+--------+--------+-------+
| 唐三藏    | 98     | 56     | 67     |
| 猪悟能    | 98     | 90     | 88     |
| 刘玄德    | 85     | 45     | 55     |
| 曹孟德    | 84     | 67     | 82     |
| 孙悟空    | 78     | 77     | 87     |
| 孙权      | 73     | 78     | 70     |
| 宋公明    | 65     | 30     | 75     |
+-----------+--------+--------+-------+
7 rows in set (0.00 sec)-- 查询同学及总分，由高到低
SELECT name, chinese + english + math AS 总分 FROM exam_result
ORDER BY 总分 DESC;
+-----------+--------+
| name      | 总分   |
+-----------+--------+
| 猪悟能    | 276    |
| 孙悟空    | 242    |
| 曹孟德    | 233    |
| 唐三藏    | 221    |
| 孙权      | 221    |
| 刘玄德    | 185    |
| 宋公明    | 170    |
+-----------+--------+
7 rows in set (0.00 sec)-- 查询姓孙的同学或者姓曹的同学数学成绩，结果按数学成绩由高到低显示
SELECT name, math FROM exam_result
WHERE name LIKE '孙%' OR name LIKE '曹%'
ORDER BY math DESC;
+-----------+--------+
| name      | math   |
+-----------+--------+
| 曹孟德    | 84     |
| 孙悟空    | 78     |
| 孙权      | 73     |
+-----------+--------+
3 rows in set (0.00 sec)
6.2.4 筛选分页结果

语法：

SELECT ... FROM table_name [WHERE ...] [ORDER BY ...] LIMIT s, n;

案例：

-- 按 id 进行分页，每页 3 条记录，分别显示 第 1、2、3 页
-- 第 1 页
SELECT id, name, math, english, chinese FROM exam_result
ORDER BY id LIMIT 3 OFFSET 0;
+----+-----------+--------+--------+-------+
| id | name      | math   | english | chinese |
+----+-----------+--------+--------+-------+
| 1  | 唐三藏    | 98     | 56     | 67     |
| 2  | 孙悟空    | 78     | 77     | 87     |
| 3  | 猪悟能    | 98     | 90     | 88     |
+----+-----------+--------+--------+-------+
3 rows in set (0.02 sec)-- 第 2 页
SELECT id, name, math, english, chinese FROM exam_result
ORDER BY id LIMIT 3 OFFSET 3;
+----+-----------+--------+--------+-------+
| id | name      | math   | english | chinese |
+----+-----------+--------+--------+-------+
| 4  | 曹孟德    | 84     | 67     | 82     |
| 5  | 刘玄德    | 85     | 45     | 55     |
| 6  | 孙权      | 73     | 78     | 70     |
+----+-----------+--------+--------+-------+
3 rows in set (0.00 sec)-- 第 3 页，如果结果不足 3 个，不会有影响
SELECT id, name, math, english, chinese FROM exam_result
ORDER BY id LIMIT 3 OFFSET 6;
+----+-----------+--------+--------+-------+
| id | name      | math   | english | chinese |
+----+-----------+--------+--------+-------+
| 7  | 宋公明    | 65     | 30     | 75     |
+----+-----------+--------+--------+-------+
1 row in set (0.00 sec)
3. Update（更新）

语法：

UPDATE table_name SET column = expr [, column = expr ...]
[WHERE ...] [ORDER BY ...] [LIMIT ...];

案例：

-- 将孙悟空同学的数学成绩变更为 80 分
SELECT name, math FROM exam_result WHERE name = '孙悟空';
+-----------+--------+
| name      | math   |
+-----------+--------+
| 孙悟空    | 78     |
+-----------+--------+
1 row in set (0.00 sec)UPDATE exam_result SET math = 80 WHERE name = '孙悟空';
Query OK, 1 row affected (0.04 sec)
Rows matched: 1 Changed: 1 Warnings: 0SELECT name, math FROM exam_result WHERE name = '孙悟空';
+-----------+--------+
| name      | math   |
+-----------+--------+
| 孙悟空    | 80     |
+-----------+--------+
1 row in set (0.00 sec)-- 将曹孟德同学的数学成绩变更为 60 分，语文成绩变更为 70 分
SELECT name, math, chinese FROM exam_result WHERE name = '曹孟德';
+-----------+--------+-------+
| name      | math   | chinese |
+-----------+--------+-------+
| 曹孟德    | 84     | 82     |
+-----------+--------+-------+
1 row in set (0.00 sec)UPDATE exam_result SET math = 60, chinese = 70 WHERE name = '曹孟德';
Query OK, 1 row affected (0.14 sec)
Rows matched: 1 Changed: 1 Warnings: 0SELECT name, math, chinese FROM exam_result WHERE name = '曹孟德';
+-----------+--------+-------+
| name      | math   | chinese |
+-----------+--------+-------+
| 曹孟德    | 60     | 70     |
+-----------+--------+-------+
1 row in set (0.00 sec)-- 将总成绩倒数前三的 3 位同学的数学成绩加上 30 分
SELECT name, math, chinese + math + english AS 总分 FROM exam_result
ORDER BY 总分 LIMIT 3;
+-----------+--------+--------+
| name      | math   | 总分   |
+-----------+--------+--------+
| 宋公明    | 65     | 170    |
| 刘玄德    | 85     | 185    |
| 曹孟德    | 60     | 197    |
+-----------+--------+--------+
3 rows in set (0.00 sec)UPDATE exam_result SET math = math + 30
ORDER BY chinese + math + english LIMIT 3;
Query OK, 3 rows affected (0.00 sec)
Rows matched: 3 Changed: 3 Warnings: 0SELECT name, math, chinese + math + english AS 总分 FROM exam_result
WHERE name IN ('宋公明', '刘玄德', '曹孟德');
+-----------+--------+--------+
| name      | math   | 总分   |
+-----------+--------+--------+
| 曹孟德    | 90     | 227    |
| 刘玄德    | 115    | 215    |
| 宋公明    | 95     | 200    |
+-----------+--------+--------+
3 rows in set (0.00 sec)SELECT name, math, chinese + math + english AS 总分 FROM exam_result
ORDER BY 总分 LIMIT 3;
+-----------+--------+--------+
| name      | math   | 总分   |
+-----------+--------+--------+
| 宋公明    | 95     | 200    |
| 刘玄德    | 115    | 215    |
| 唐三藏    | 98     | 221    |
+-----------+--------+--------+
3 rows in set (0.00 sec)-- 将所有同学的语文成绩更新为原来的 2 倍
SELECT * FROM exam_result;
+----+-----------+-------+--------+--------+
| id | name      | chinese | math   | english |
+----+-----------+-------+--------+--------+
| 1  | 唐三藏    | 67     | 98     | 56     |
| 2  | 孙悟空    | 87     | 80     | 77     |
| 3  | 猪悟能    | 88     | 98     | 90     |
| 4  | 曹孟德    | 70     | 90     | 67     |
| 5  | 刘玄德    | 55     | 115    | 45     |
| 6  | 孙权      | 70     | 73     | 78     |
| 7  | 宋公明    | 75     | 95     | 30     |
+----+-----------+-------+--------+--------+
7 rows in set (0.00 sec)UPDATE exam_result SET chinese = chinese * 2;
Query OK, 7 rows affected (0.00 sec)
Rows matched: 7 Changed: 7 Warnings: 0SELECT * FROM exam_result;
+----+-----------+-------+--------+--------+
| id | name      | chinese | math   | english |
+----+-----------+-------+--------+--------+
| 1  | 唐三藏    | 134    | 98     | 56     |
| 2  | 孙悟空    | 174    | 80     | 77     |
| 3  | 猪悟能    | 176    | 98     | 90     |
| 4  | 曹孟德    | 140    | 90     | 67     |
| 5  | 刘玄德    | 110    | 115    | 45     |
| 6  | 孙权      | 140    | 73     | 78     |
| 7  | 宋公明    | 150    | 95     | 30     |
+----+-----------+-------+--------+--------+
7 rows in set (0.00 sec)
4. Delete（删除）
4.1 删除数据

语法：

DELETE FROM table_name [WHERE ...] [ORDER BY ...] [LIMIT ...];

案例：

-- 删除孙悟空同学的考试成绩
SELECT * FROM exam_result WHERE name = '孙悟空';
+----+-----------+-------+--------+--------+
| id | name      | chinese | math   | english |
+----+-----------+-------+--------+--------+
| 2  | 孙悟空    | 174    | 80     | 77     |
+----+-----------+-------+--------+--------+
1 row in set (0.00 sec)DELETE FROM exam_result WHERE name = '孙悟空';
Query OK, 1 row affected (0.17 sec)SELECT * FROM exam_result WHERE name = '孙悟空';
Empty set (0.00 sec)-- 删除整张表数据
CREATE TABLE for_delete (id INT PRIMARY KEY AUTO_INCREMENT,name VARCHAR(20)
);
Query OK, 0 rows affected (0.16 sec)INSERT INTO for_delete (name) VALUES ('A'), ('B'), ('C');
Query OK, 3 rows affected (1.05 sec)
Records: 3 Duplicates: 0 Warnings: 0SELECT * FROM for_delete;
+----+------+
| id | name |
+----+------+
| 1  | A    |
| 2  | B    |
| 3  | C    |
+----+------+
3 rows in set (0.00 sec)DELETE FROM for_delete;
Query OK, 3 rows affected (0.00 sec)SELECT * FROM for_delete;
Empty set (0.00 sec)INSERT INTO for_delete (name) VALUES ('D');
Query OK, 1 row affected (0.00 sec)SELECT * FROM for_delete;
+----+------+
| id | name |
+----+------+
| 4  | D    |
+----+------+
1 row in set (0.00 sec)SHOW CREATE TABLE for_delete\G
*************************** 1. row ***************************
Table: for_delete
Create Table: CREATE TABLE `for_delete` (`id` int(11) NOT NULL AUTO_INCREMENT,`name` varchar(20) DEFAULT NULL,PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8
1 row in set (0.00 sec)
4.2 截断表

语法：

TRUNCATE [TABLE] table_name;

案例：

-- 截断表
CREATE TABLE for_truncate (id INT PRIMARY KEY AUTO_INCREMENT,name VARCHAR(20)
);
Query OK, 0 rows affected (0.16 sec)INSERT INTO for_truncate (name) VALUES ('A'), ('B'), ('C');
Query OK, 3 rows affected (1.05 sec)
Records: 3 Duplicates: 0 Warnings: 0SELECT * FROM for_truncate;
+----+------+
| id | name |
+----+------+
| 1  | A    |
| 2  | B    |
| 3  | C    |
+----+------+
3 rows in set (0.00 sec)TRUNCATE for_truncate;
Query OK, 0 rows affected (0.10 sec)SELECT * FROM for_truncate;
Empty set (0.00 sec)INSERT INTO for_truncate (name) VALUES ('D');
Query OK, 1 row affected (0.00 sec)SELECT * FROM for_truncate;
+----+------+
| id | name |
+----+------+
| 1  | D    |
+----+------+
1 row in set (0.00 sec)SHOW CREATE TABLE for_truncate\G
*************************** 1. row ***************************
Table: for_truncate
Create Table: CREATE TABLE `for_truncate` (`id` int(11) NOT NULL AUTO_INCREMENT,`name` varchar(20) DEFAULT NULL,PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8
1 row in set (0.00 sec)
5. 插入查询结果

语法：

INSERT INTO table_name [(column [, column ...])] SELECT ...;

案例：

-- 删除表中的重复记录，重复的数据只能有一份
CREATE TABLE duplicate_table (id int, name varchar(20));
Query OK, 0 rows affected (0.01 sec)INSERT INTO duplicate_table VALUES
(100, 'aaa'),
(100, 'aaa'),
(200, 'bbb'),
(200, 'bbb'),
(200, 'bbb'),
(300, 'ccc');
Query OK, 6 rows affected (0.00 sec)
Records: 6 Duplicates: 0 Warnings: 0-- 创建一张空表 no_duplicate_table，结构和 duplicate_table 一样
CREATE TABLE no_duplicate_table LIKE duplicate_table;
Query OK, 0 rows affected (0.00 sec)-- 将 duplicate_table 的去重数据插入到 no_duplicate_table
INSERT INTO no_duplicate_table SELECT DISTINCT * FROM duplicate_table;
Query OK, 3 rows affected (0.00 sec)
Records: 3 Duplicates: 0 Warnings: 0-- 通过重命名表，实现原子的去重操作
RENAME TABLE duplicate_table TO old_duplicate_table,no_duplicate_table TO duplicate_table;
Query OK, 0 rows affected (0.00 sec)-- 查看最终结果
SELECT * FROM duplicate_table;
+-----+------+
| id  | name |
+-----+------+
| 100 | aaa  |
| 200 | bbb  |
| 300 | ccc  |
+-----+------+
3 rows in set (0.00 sec)
6. 聚合函数

案例：

-- 统计班级共有多少同学
SELECT COUNT(*) FROM students;
+----------+
| COUNT(*) |
+----------+
| 4        |
+----------+
1 row in set (0.00 sec)-- 统计班级收集的 qq 号有多少
SELECT COUNT(qq) FROM students;
+-----------+
| COUNT(qq) |
+-----------+
| 1         |
+-----------+
1 row in set (0.00 sec)-- 统计本次考试的数学成绩分数个数
SELECT COUNT(math) FROM exam_result;
+---------------+
| COUNT(math)   |
+---------------+
| 6             |
+---------------+
1 row in set (0.00 sec)SELECT COUNT(DISTINCT math) FROM exam_result;
+------------------------+
| COUNT(DISTINCT math)   |
+------------------------+
| 5                      |
+------------------------+
1 row in set (0.00 sec)-- 统计数学成绩总分
SELECT SUM(math) FROM exam_result;
+-------------+
| SUM(math)   |
+-------------+
| 569         |
+-------------+
1 row in set (0.00 sec)-- 不及格 < 60 的总分，没有结果，返回 NULL
SELECT SUM(math) FROM exam_result WHERE math < 60;
+-------------+
| SUM(math)   |
+-------------+
| NULL        |
+-------------+
1 row in set (0.00 sec)-- 统计平均总分
SELECT AVG(chinese + math + english) AS 平均总分 FROM exam_result;
+--------------+
| 平均总分     |
+--------------+
| 297.5        |
+--------------+
1 row in set (0.00 sec)-- 返回英语最高分
SELECT MAX(english) FROM exam_result;
+-------------+
| MAX(english)|
+-------------+
| 90          |
+-------------+
1 row in set (0.00 sec)-- 返回 > 70 分以上的数学最低分
SELECT MIN(math) FROM exam_result WHERE math > 70;
+-------------+
| MIN(math)   |
+-------------+
| 73          |
+-------------+
1 row in set (0.00 sec)
7. GROUP BY 子句的使用

在 SELECT 中使用 GROUP BY 子句可以对指定列进行分组查询。

语法：

SELECT column1, column2, ... FROM table GROUP BY column;

案例：

-- 显示每个部门的平均工资和最高工资
SELECT deptno, AVG(sal), MAX(sal) FROM EMP GROUP BY deptno;-- 显示每个部门的每种岗位的平均工资和最低工资
SELECT AVG(sal), MIN(sal), job, deptno FROM EMP GROUP BY deptno, job;-- 显示平均工资低于 2000 的部门和它的平均工资
SELECT AVG(sal) AS myavg FROM EMP GROUP BY deptno HAVING myavg < 2000;
8. 实战 OJ

  • 牛客：批量插入数据

  • 牛客：找出所有员工当前 (to_date='9999-01-01') 具体的薪水 salary 情况，对于相同的薪水只显示一次，并按照逆序显示

  • 牛客：查找最晚入职员工的所有信息

  • 牛客：查找入职员工时间排名倒数第三的员工所有信息

  • 牛客：查找薪水涨幅超过 15 次的员工号 emp_no 以及其对应的涨幅次数 t

  • 牛客：获取所有部门当前 manager 的当前薪水情况，给出 dept_no, emp_no 以及 salary，当前表示 to_date='9999-01-01'

  • 牛客：从 titles 表获取按照 title 进行分组，每组个数大于等于 2，给出 title 以及对应的数目 t

LeetCode：

  • duplicate-emails

  • big-countries

  • nth-highest-salary

面试题：SQL 查询中各个关键字的执行先后顺序

FROM > ON > JOIN > WHERE > GROUP BY > WITH > HAVING > SELECT > DISTINCT > ORDER BY > LIMIT

from > on > join > where > group by > with > having > select > distinct > order by > limit

  1. FROM：首先指定要查询的表。

  2. ON：在多表连接时，用于指定连接条件。

  3. JOIN：用于表之间的连接操作。

  4. WHERE：对数据进行筛选，过滤不符合条件的行。

  5. GROUP BY：对结果集进行分组。

  6. HAVING：对分组后的结果进行筛选。

  7. SELECT：指定要查询的列。

  8. DISTINCT：用于去除重复的行。

  9. ORDER BY：对结果集进行排序。

  10. LIMIT：限制返回的行数。

需要注意的是，WITH 是一个子查询的语法，它在逻辑上可以看作是在查询的最开始执行的，用于定义临时的表或视图，但并不影响上述关键字的执行顺序。

在实际的 SQL 查询中，数据库引擎会根据查询的复杂性和优化器的规则对查询进行优化，但上述顺序是逻辑上的执行顺序。

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