MySQL表的增删改查(CRUD)
MySQL表的增删改查(CRUD)
文章目录
- MySQL表的增删改查(CRUD)
- 1. Create
- 1.1 单行数据 + 全列插入
- 1.2 多行数据 + 指定列插入
- 1.3 插入否则更新
- 1.4 替换
- 2. Retrieve
- 2.1 SELECT 列
- 2.1.1 全列查询
- 2.1.2 指定列查询
- 2.1.3 查询字段为表达式
- 2.1.4 为查询指定别名
- 2.1.5 结果去重
- 2.2 WHERE 条件
- 2.2.1 英语不及格的同学(英语成绩 80 并且不姓孙的同学
- 2.2.8 孙某同学,否则要求总成绩 > 200 并且 语文成绩 80
- 2.2.9 NULL 的查询
- 2.3 结果排序
- 2.3.1 同学及数学成绩,按数学成绩升序显示
- 2.3.2 同学及 qq 号,按 qq 号排序显示
- 2.3.3 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
- 2.3.4 查询同学及总分,由高到低
- 2.3.5 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
- 2.4 筛选分页结果
- 3. Update
- 3.1 单列变更
- 3.2 一次更新多个列
- 3.3 更新值为原值基础上进行变更
- 3.4 将所有同学的语文成绩更新为原来的 2 倍
- 4. Delete
- 4.1 删除数据
- 4.1.1 删除孙悟空同学的考试成绩
- 4.1.2 删除整张表数据
- 4.2 截断表
- 5. 插入查询结果(insert into... select...)
- 6. 聚合函数
- 6.1 统计班级共有多少同学
- 6.2 统计班级收集的 qq 号有多少
- 6.3 统计本次考试的数学成绩分数个数
- 6.4 统计数学成绩总分
- 6.5 统计平均总分
- 6.6 返回英语最高分
- 6.7 返回 > 70 分以上的数学最低分
- 7. group by子句的使用
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
1. Create
语法:
INSERT [INTO] table_name [(column [, column] ...)] VALUES (value_list) [, (value_list)] ... value_list: value, [, value] ...
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) );
1.1 单行数据 + 全列插入
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。
mysql> INSERT INTO students VALUES (100, 10000, '唐三藏', NULL); Query OK, 1 row affected (0.00 sec) mysql> INSERT INTO students VALUES (101, 10001, '孙悟空', '11111'); Query OK, 1 row affected (0.01 sec) mysql> 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)
注意,这里在插入的时候,也可以不用指定id(当然,那时候就需要明确插入数据到那些列了),那么mysql会使用默认的值进行自增。
1.2 多行数据 + 指定列插入
插入两条记录,value_list 数量必须和指定列数量及顺序一致
mysql> INSERT INTO students (id, sn, name) VALUES -> (102, 20001, '曹孟德'), -> (103, 20002, '孙仲谋'); Query OK, 2 rows affected (0.00 sec) Records: 2 Duplicates: 0 Warnings: 0 mysql> 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
由于 主键 或者 唯一键 对应的值已经存在而导致插入失败
# 主键冲突 mysql> INSERT INTO students (id, sn, name) VALUES (100, 10010, '唐大师'); ERROR 1062 (23000): Duplicate entry '100' for key 'students.PRIMARY' # 唯一键冲突 mysql> INSERT INTO students (sn, name) VALUES (20001, '曹阿瞒'); ERROR 1062 (23000): Duplicate entry '20001' for key 'students.sn'
可以选择性的进行同步更新操作
语法:
INSERT ... ON DUPLICATE KEY UPDATE column = value [, column = value] ...
ON DUPLICATE KEY 当发生重复key的时候
示例:
mysql> 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) -- 0 row affected: 表中有冲突数据,但冲突数据的值和 update 的值相等 -- 1 row affected: 表中没有冲突数据,数据被插入 -- 2 row affected: 表中有冲突数据,并且数据已经被更新 mysql> SELECT ROW_COUNT(); +-------------+ | ROW_COUNT() | +-------------+ | -1 | +-------------+ 1 row in set (0.00 sec)
1.4 替换
# 主键 或者 唯一键 没有冲突,则直接插入; # 主键 或者 唯一键 如果冲突,则删除后再插入 REPLACE INTO students (sn, name) VALUES (20001, '曹阿瞒'); Query OK, 2 rows affected (0.00 sec) -- 1 row affected: 表中没有冲突数据,数据被插入 -- 2 row affected: 表中有冲突数据,删除后重新插入
2. Retrieve
语法:
SELECT [DISTINCT] {* | {column [, column] ...} [FROM table_name] [WHERE ...] [ORDER BY column [ASC | DESC], ...] LIMIT ...
案例前置数据导入:
mysql> 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' -> ); Query OK, 0 rows affected (0.02 sec) mysql> 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 (0.01 sec) Records: 7 Duplicates: 0 Warnings: 0
2.1 SELECT 列
2.1.1 全列查询
语法:
SELECT * FROM 表名;
通常情况下不建议使用 * 进行全列查询
-
查询的列越多,意味着需要传输的数据量越大;
-
可能会影响到索引的使用。
案例:
mysql> 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)
2.1.2 指定列查询
指定列的顺序不需要按定义表的顺序
mysql> 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) mysql> SELECT id, math, name FROM exam_result; +----+------+-----------+ | id | math | name | +----+------+-----------+ | 1 | 98 | 唐三藏 | | 2 | 78 | 孙悟空 | | 3 | 98 | 猪悟能 | | 4 | 84 | 曹孟德 | | 5 | 85 | 刘玄德 | | 6 | 73 | 孙权 | | 7 | 65 | 宋公明 | +----+------+-----------+ 7 rows in set (0.00 sec)
2.1.3 查询字段为表达式
- 表达式不包含字段
mysql> 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 (0.01 sec)
- 表达式包含一个字段
mysql> 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 (0.01 sec)
- 表达式包含多个字段
mysql> 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 (0.01 sec)
2.1.4 为查询指定别名
语法:
SELECT column [AS] alias_name [...] FROM table_name;
案例:
mysql> SELECT id, name, chinese + math + english 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)
2.1.5 结果去重
# 查询发现数学成绩有重复的 mysql> SELECT math FROM exam_result; +------+ | math | +------+ | 98 | | 78 | | 98 | | 84 | | 85 | | 73 | | 65 | +------+ 7 rows in set (0.00 sec) # 利用DISTINCT关键字去重 mysql> SELECT DISTINCT math FROM exam_result; +------+ | math | +------+ | 98 | | 78 | | 84 | | 85 | | 73 | | 65 | +------+ 6 rows in set (0.03 sec)
2.2 WHERE 条件
比较运算符:
逻辑运算符:
案例:
2.2.1 英语不及格的同学(英语成绩 SELECT name, english FROM exam_result WHERE english
2.2.2 语文成绩在 [80, 90] 分的同学及语文成绩
# 使用 and 条件连接语句 mysql> SELECT name, chinese FROM exam_result WHERE chinese >= 80 AND chinese 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 (0.02 sec)
2.2.3 数学成绩是 58 或者 59 或者 98 或者 99 分的同学及数学成绩
# 使用 or 条件连接语句 mysql> SELECT name, math FROM exam_result -> WHERE math = 58 -> OR math = 59 -> OR math = 98 -> OR math = 99; +-----------+------+ | name | math | +-----------+------+ | 唐三藏 | 98 | | 猪悟能 | 98 | +-----------+------+ 2 rows in set (0.00 sec) # 使用 in 语句 mysql> 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)
2.2.4 姓孙的同学 及 孙某同学
- % 匹配任意多个(包括 0 个)任意字符
mysql> SELECT name FROM exam_result WHERE name LIKE '孙%'; +-----------+ | name | +-----------+ | 孙悟空 | | 孙权 | +-----------+ 2 rows in set (0.01 sec)
- _ 匹配严格的一个任意字符
mysql> SELECT name FROM exam_result WHERE name LIKE '孙_'; +--------+ | name | +--------+ | 孙权 | +--------+ 1 row in set (0.00 sec)
2.2.5 语文成绩好于英语成绩的同学
WHERE 条件中比较运算符两侧都是字段
mysql> 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)
2.2.6 总分在 200 分以下的同学
WHERE 条件中使用表达式
# 别名不能用在 WHERE 条件中 mysql> SELECT name, chinese + math + english 总分 FROM exam_result -> WHERE chinese + math + english
2.2.7 语文成绩 > 80 并且不姓孙的同学
# and 与 not 的使用 mysql> 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)
2.2.8 孙某同学,否则要求总成绩 > 200 并且 语文成绩 80
# 综合查询 mysql> select name, chinese, math, english, chinese+math+english total -> from exam_result where -> (name like '孙%') or -> (chinese+math+english > 200 and chinese 80); +-----------+---------+------+---------+-------+ | name | chinese | math | english | total | +-----------+---------+------+---------+-------+ | 孙悟空 | 87 | 78 | 77 | 242 | | 猪悟能 | 88 | 98 | 90 | 276 | | 孙权 | 70 | 73 | 78 | 221 | +-----------+---------+------+---------+-------+ 3 rows in set (0.00 sec)
2.2.9 NULL 的查询
# 查询所有学生 mysql> select * from students; +-----+-------+-----------+-------+ | id | sn | name | qq | +-----+-------+-----------+-------+ | 100 | 10010 | 唐大师 | NULL | | 101 | 10001 | 孙悟空 | 11111 | | 103 | 20002 | 孙仲谋 | NULL | | 105 | 20001 | 曹阿瞒 | NULL | +-----+-------+-----------+-------+ 4 rows in set (0.01 sec) # 查询 qq 非空的学生 mysql> SELECT name, qq FROM students WHERE qq IS NOT NULL; +-----------+-------+ | name | qq | +-----------+-------+ | 孙悟空 | 11111 | +-----------+-------+ 1 row in set (0.00 sec) # = 和 对NULL查询的影响 mysql> SELECT NULL = NULL, NULL = 1, NULL = 0; +-------------+----------+----------+ | NULL = NULL | NULL = 1 | NULL = 0 | +-------------+----------+----------+ | NULL | NULL | NULL | +-------------+----------+----------+ 1 row in set (0.00 sec) mysql> SELECT NULL NULL, NULL 1, NULL 0; +---------------+------------+------------+ | NULL NULL | NULL 1 | NULL 0 | +---------------+------------+------------+ | 1 | 0 | 0 | +---------------+------------+------------+ 1 row in set (0.00 sec)
2.3 结果排序
- ASC 为升序(从小到大)
- DESC 为降序(从大到小)
- 默认为 ASC
语法:
SELECT ... FROM table_name [WHERE ...] ORDER BY column [ASC|DESC], [...];
注意:没有 ORDER BY 子句的查询,返回的顺序是未定义的,永远不要依赖这个顺序
案例:
2.3.1 同学及数学成绩,按数学成绩升序显示
mysql> 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)
2.3.2 同学及 qq 号,按 qq 号排序显示
# NULL 视为比任何值都小,升序出现在最上面 mysql> 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 mysql> 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)
2.3.3 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
多字段排序,排序优先级随书写顺序
# 优先数学降序,数学字段对应值相同时,英语升序... mysql> 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)
2.3.4 查询同学及总分,由高到低
# order by 中可以使用表达式 mysql> SELECT name, chinese + english + math FROM exam_result -> ORDER BY chinese + english + math DESC; +-----------+--------------------------+ | name | chinese + english + math | +-----------+--------------------------+ | 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) # ORDER BY 子句中可以使用列别名 mysql> SELECT name, chinese + english + math 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)
2.3.5 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
# 结合 WHERE 子句和 ORDER BY 子句 mysql> 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)
2.4 筛选分页结果
语法:
-- 起始下标为 0 -- 从 s 开始,筛选 n 条结果 SELECT ... FROM table_name [WHERE ...] [ORDER BY ...] LIMIT s, n -- 从0 开始,筛选n 条结果 SELECT ... FROM table_name [WHERE ...] [ORDER BY ...] LIMIT n; ; -- 从 s 开始,筛选 n 条结果,比第二种用法更明确,建议使用 SELECT ... FROM table_name [WHERE ...] [ORDER BY ...] LIMIT n OFFSET s;
建议:对未知表进行查询时,最好加一条 LIMIT 1,避免因为表中数据过大,查询全表数据导致数据库卡死
案例:
按 id 进行分页,每页 3 条记录,分别显示 第 1、2、3 页
# 第一页 mysql> 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 (0.01 sec) # 第二页 mysql> 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) # 第三页 mysql> 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
语法:
UPDATE table_name SET column = expr [, column = expr ...] [WHERE ...] [ORDER BY ...] [LIMIT ...]
对查询到的结果进行列值更新
案例:
3.1 单列变更
将孙悟空同学的数学成绩变更为 80 分
# 查看原数据 mysql> SELECT name, math FROM exam_result WHERE name = '孙悟空'; +-----------+------+ | name | math | +-----------+------+ | 孙悟空 | 78 | +-----------+------+ 1 row in set (0.01 sec) # 数据更新 mysql> UPDATE exam_result SET math = 80 WHERE name = '孙悟空'; Query OK, 1 row affected (0.00 sec) Rows matched: 1 Changed: 1 Warnings: 0 # 查看更新后数据 mysql> SELECT name, math FROM exam_result WHERE name = '孙悟空'; +-----------+------+ | name | math | +-----------+------+ | 孙悟空 | 80 | +-----------+------+ 1 row in set (0.00 sec)
3.2 一次更新多个列
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
# 查看原数据 mysql> 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) # 数据更新 mysql> UPDATE exam_result SET math = 60, chinese = 70 WHERE name = '曹孟德'; Query OK, 1 row affected (0.00 sec) Rows matched: 1 Changed: 1 Warnings: 0 # 查看更新后数据 mysql> SELECT 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)
3.3 更新值为原值基础上进行变更
将总成绩倒数前三的 3 位同学的数学成绩加上 30 分
# 查看原数据,别名可以在 ORDER BY 中使用 mysql> SELECT name, math, chinese + math + english 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) # 数据更新 mysql> 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: 0 # 查询更新后的信息 mysql> SELECT name, math, chinese + math + english 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) # mysql> SELECT name, math, chinese + math + english 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) mysql> SELECT name, math, chinese + math + english 总分 FROM exam_result -> WHERE name IN ('宋公明', '刘玄德', '曹孟德') ORDER BY 总分 desc; +-----------+------+--------+ | 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)
3.4 将所有同学的语文成绩更新为原来的 2 倍
注意:更新全表语句慎用
- 没有 WHERE 子句,更新全表
# 查看原数据 mysql> 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 (0.01 sec) # 全表数据更新 mysql> UPDATE exam_result SET chinese = chinese * 2; Query OK, 7 rows affected (0.06 sec) Rows matched: 7 Changed: 7 Warnings: 0 # 查看更新后数据 mysql> SELECT * 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
4.1 删除数据
语法:
DELETE FROM table_name [WHERE ...] [ORDER BY ...] [LIMIT ...]
案例:
4.1.1 删除孙悟空同学的考试成绩
# 查看原数据 mysql> 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) # 删除数据 mysql> DELETE FROM exam_result WHERE name = '孙悟空'; Query OK, 1 row affected (0.00 sec) # 查看删除结果 mysql> SELECT * FROM exam_result WHERE name = '孙悟空'; Empty set (0.00 sec)
4.1.2 删除整张表数据
注意:删除整表操作要慎用!
前置准备:
mysql> CREATE TABLE for_delete ( id INT PRIMARY KEY AUTO_INCREMENT, name VARCHAR(20) ); Query OK, 0 rows affected (0.00 sec) mysql> INSERT INTO for_delete (name) VALUES ('A'), ('B'), ('C'); Query OK, 3 rows affected (0.01 sec) Records: 3 Duplicates: 0 Warnings: 0 mysql> SELECT * FROM for_delete; +----+------+ | id | name | +----+------+ | 1 | A | | 2 | B | | 3 | C | +----+------+ 3 rows in set (0.00 sec)
删除操作:
mysql> DELETE FROM for_delete; Query OK, 3 rows affected (0.00 sec) mysql> SELECT * FROM for_delete; Empty set (0.00 sec)
再插入一条数据,自增 id 在原值上增长
# 插入数据 mysql> INSERT INTO for_delete (name) VALUES ('D'); Query OK, 1 row affected (0.00 sec) mysql> SELECT * FROM for_delete; +----+------+ | id | name | +----+------+ | 4 | D | +----+------+ 1 row in set (0.00 sec) # 查看表结构,会有 AUTO_INCREMENT=n 项 mysql> SHOW CREATE TABLE for_delete\G; *************************** 1. row *************************** Table: for_delete Create Table: CREATE TABLE `for_delete` ( `id` int NOT NULL AUTO_INCREMENT, `name` varchar(20) DEFAULT NULL, PRIMARY KEY (`id`) ) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci 1 row in set (0.00 sec) ERROR: No query specified
4.2 截断表
语法:
TRUNCATE [TABLE] table_name
注意:这个操作慎用
- 只能对整表操作,不能像 DELETE 一样针对部分数据操作;
- 实际上 MySQL 不对数据操作,所以比 DELETE 更快,但是TRUNCATE在删除数据的时候,并不经过真正的事务,所以无法回滚
- 会重置 AUTO_INCREMENT 项
前置准备:
mysql> CREATE TABLE for_truncate ( -> id INT PRIMARY KEY AUTO_INCREMENT, -> name VARCHAR(20) -> ); Query OK, 0 rows affected (0.01 sec) mysql> INSERT INTO for_truncate (name) VALUES ('A'), ('B'), ('C'); Query OK, 3 rows affected (0.00 sec) Records: 3 Duplicates: 0 Warnings: 0 mysql> SELECT * FROM for_truncate; +----+------+ | id | name | +----+------+ | 1 | A | | 2 | B | | 3 | C | +----+------+ 3 rows in set (0.00 sec)
截断操作:
mysql> TRUNCATE for_truncate; Query OK, 0 rows affected (0.03 sec) mysql> SELECT * FROM for_truncate; Empty set (0.00 sec)
再插入一条数据,自增 id 在重新增长:
# 插入数据 mysql> INSERT INTO for_truncate (name) VALUES ('D'); Query OK, 1 row affected (0.00 sec) mysql> SELECT * FROM for_truncate; +----+------+ | id | name | +----+------+ | 1 | D | +----+------+ 1 row in set (0.00 sec) # 查看表结构,会有 AUTO_INCREMENT=2 项 mysql> SHOW CREATE TABLE for_truncate\G; *************************** 1. row *************************** Table: for_truncate Create Table: CREATE TABLE `for_truncate` ( `id` int NOT NULL AUTO_INCREMENT, `name` varchar(20) DEFAULT NULL, PRIMARY KEY (`id`) ) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci 1 row in set (0.00 sec) ERROR: No query specified
5. 插入查询结果(insert into… select…)
语法:
INSERT INTO table_name [(column [, column ...])] SELECT ...
案例:
删除表中的的重复复记录,重复的数据只能有一份
mysql> CREATE TABLE duplicate_table (id int, name varchar(20)); Query OK, 0 rows affected (0.01 sec) mysql> INSERT INTO duplicate_table VALUES -> (100, 'aaa'), -> (100, 'aaa'), -> (200, 'bbb'), -> (200, 'bbb'), -> (200, 'bbb'), -> (300, 'ccc'); Query OK, 6 rows affected (0.02 sec) Records: 6 Duplicates: 0 Warnings: 0
思路:
# 创建一张空表 no_duplicate_table,结构和 duplicate_table 一样 mysql> 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 mysql> INSERT INTO no_duplicate_table SELECT DISTINCT * FROM duplicate_table; Query OK, 3 rows affected (0.01 sec) Records: 3 Duplicates: 0 Warnings: 0 # 通过重命名表,实现原子的去重操作 mysql> RENAME TABLE duplicate_table TO old_duplicate_table, -> no_duplicate_table TO duplicate_table; Query OK, 0 rows affected (0.04 sec) # 查看最终结果 mysql> 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:
6.1 统计班级共有多少同学
# 使用 * 做统计,不受 NULL 影响 mysql> SELECT COUNT(*) FROM students; +----------+ | COUNT(*) | +----------+ | 4 | +----------+ 1 row in set (0.01 sec) # 使用表达式做统计 mysql> SELECT COUNT(1) FROM students; +----------+ | COUNT(1) | +----------+ | 4 | +----------+ 1 row in set (0.00 sec)
6.2 统计班级收集的 qq 号有多少
# NULL 不会计入结果 mysql> SELECT COUNT(qq) FROM students; +-----------+ | COUNT(qq) | +-----------+ | 1 | +-----------+ 1 row in set (0.02 sec)
6.3 统计本次考试的数学成绩分数个数
# 查看所有信息 mysql> SELECT * FROM exam_result; +----+-----------+---------+------+---------+ | id | name | chinese | math | english | +----+-----------+---------+------+---------+ | 1 | 唐三藏 | 134 | 98 | 56 | | 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 | +----+-----------+---------+------+---------+ 6 rows in set (0.00 sec) # COUNT(math) 统计的是全部成绩 mysql> SELECT COUNT(math) FROM exam_result; +-------------+ | COUNT(math) | +-------------+ | 6 | +-------------+ 1 row in set (0.00 sec) # COUNT(DISTINCT math) 统计的是去重成绩数量 mysql> SELECT COUNT(DISTINCT math) FROM exam_result; +----------------------+ | COUNT(DISTINCT math) | +----------------------+ | 5 | +----------------------+ 1 row in set (0.00 sec)
6.4 统计数学成绩总分
mysql> SELECT SUM(math) FROM exam_result; +-----------+ | SUM(math) | +-----------+ | 569 | +-----------+ 1 row in set (0.00 sec) # 不及格 SELECT SUM(math) FROM exam_result WHERE math
6.5 统计平均总分
mysql> SELECT AVG(chinese + math + english) 平均总分 FROM exam_result; +--------------+ | 平均总分 | +--------------+ | 297.5 | +--------------+ 1 row in set (0.00 sec)
6.6 返回英语最高分
mysql> SELECT MAX(english) FROM exam_result; +--------------+ | MAX(english) | +--------------+ | 90 | +--------------+ 1 row in set (0.00 sec)
6.7 返回 > 70 分以上的数学最低分
mysql> 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
select column1, column2, .. from table group by column;
案例:
-
准备工作,创建一个雇员信息表(来自oracle 9i的经典测试表)
-
emp 员工表
-
dept 部门表
-
salgrade 工资等级表
-
如何显示每个部门的平均工资和最高工资
mysql> select deptno,avg(sal),max(sal) from emp group by deptno; +--------+-------------+----------+ | deptno | avg(sal) | max(sal) | +--------+-------------+----------+ | 20 | 2175.000000 | 3000.00 | | 30 | 1566.666667 | 2850.00 | | 10 | 2916.666667 | 5000.00 | +--------+-------------+----------+ 3 rows in set (0.00 sec)
- 显示每个部门的每种岗位的平均工资和最低工资
mysql> select avg(sal),min(sal),job, deptno from emp group by deptno, job; +-------------+----------+-----------+--------+ | avg(sal) | min(sal) | job | deptno | +-------------+----------+-----------+--------+ | 950.000000 | 800.00 | CLERK | 20 | | 1400.000000 | 1250.00 | SALESMAN | 30 | | 2975.000000 | 2975.00 | MANAGER | 20 | | 2850.000000 | 2850.00 | MANAGER | 30 | | 2450.000000 | 2450.00 | MANAGER | 10 | | 3000.000000 | 3000.00 | ANALYST | 20 | | 5000.000000 | 5000.00 | PRESIDENT | 10 | | 950.000000 | 950.00 | CLERK | 30 | | 1300.000000 | 1300.00 | CLERK | 10 | +-------------+----------+-----------+--------+ 9 rows in set (0.00 sec)
- 显示平均工资低于2000的部门和它的平均工资
mysql> select deptno, avg(sal) as 平均工资 from emp group by deptno having (平均工资
- 显示平均工资低于2000的部门和它的平均工资
- 显示每个部门的每种岗位的平均工资和最低工资
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- 没有 WHERE 子句,更新全表
- _ 匹配严格的一个任意字符
- % 匹配任意多个(包括 0 个)任意字符
- 表达式包含多个字段
- 表达式包含一个字段
-