三篇学会MySQL数据库【查询详解】

无水印脑图


表结构

mysql> desc stus;
+---------+--------------+------+-----+---------+----------------+
| Field   | Type         | Null | Key | Default | Extra          |
+---------+--------------+------+-----+---------+----------------+
| id      | int(11)      | NO   | PRI | NULL    | auto_increment |
| gradeId | int(11)      | YES  | MUL | NULL    |                |
| NAME    | varchar(100) | NO   |     | NULL    |                |
| age     | int(3)       | YES  |     | NULL    |                |
| bir     | date         | YES  |     | NULL    |                |
| english | int(3)       | YES  |     | NULL    |                |
| chinese | int(3)       | YES  |     | NULL    |                |
+---------+--------------+------+-----+---------+----------------+

mysql> select * from stus;
+----+---------+--------------+------+------------+---------+---------+
| id | gradeId | NAME         | age  | bir        | english | chinese |
+----+---------+--------------+------+------------+---------+---------+
|  1 |       1 | 周棋洛       |   18 | 2001-06-01 |      78 |    NULL |
|  2 |       2 | 张郁苗       |   18 | 2002-11-07 |     130 |     120 |
|  3 |       1 | 小猪佩奇     |    6 | 2015-06-10 |      34 |    NULL |
|  4 |       3 | 猪爸爸       |    8 | 2012-09-12 |      34 |      56 |
|  5 |       3 | 猪妈妈       |    7 | 2012-09-11 |      56 |      78 |
+----+---------+--------------+------+------------+---------+---------+

简单查询

脑图


单列查询

select 列名 from 表;

从stus表中查询名字的字段

mysql> select name from stus;
+--------------+
| name         |
+--------------+
| 周棋洛       |
| 张郁苗       |
| 小猪佩奇     |
| 猪爸爸       |
| 猪妈妈       |
+--------------+

多列查询

select 列名,列名…… from 表;

从stus表中查询名字和年龄这两个字段

mysql> select name,age from stus;
+--------------+------+
| name         | age  |
+--------------+------+
| 周棋洛       |   18 |
| 张郁苗       |   18 |
| 小猪佩奇     |    6 |
| 猪爸爸       |    8 |
| 猪妈妈       |    7 |
+--------------+------+

查询所有列 *

select * from 表;

从stus表查询所有字段

mysql> select * from stus;
+----+---------+--------------+------+------------+---------+---------+
| id | gradeId | NAME         | age  | bir        | english | chinese |
+----+---------+--------------+------+------------+---------+---------+
|  1 |       1 | 周棋洛       |   18 | 2001-06-01 |      78 |    NULL |
|  2 |       2 | 张郁苗       |   18 | 2002-11-07 |     130 |     120 |
|  3 |       1 | 小猪佩奇     |    6 | 2015-06-10 |      34 |    NULL |
|  4 |       3 | 猪爸爸       |    8 | 2012-09-12 |      34 |      56 |
|  5 |       3 | 猪妈妈       |    7 | 2012-09-11 |      56 |      78 |
+----+---------+--------------+------+------------+---------+---------+

distinct去重

distinct关键字可以出去重复出现的内容,注意:不能部分使用distinct

mysql> select distinct age from stus;
+------+
| age  |
+------+
|   18 |
|    6 |
|    8 |
|    7 |
+------+

起别名 as

我们在查询时,如果对原有字段名不满意,这时就可以通过as关键字对字段起别名,注意:as是可省略的,如下所示

mysql> select name as "姓名" from stus;
+--------------+
| 姓名         |
+--------------+
| 周棋洛       |
| 张郁苗       |
| 小猪佩奇     |
| 猪爸爸       |
| 猪妈妈       |
+--------------+

mysql> select name "姓名" from stus;
+--------------+
| 姓名         |
+--------------+
| 周棋洛       |
| 张郁苗       |
| 小猪佩奇     |
| 猪爸爸       |
| 猪妈妈       |
+--------------+

条件查询

大于判断 >

? : 查找stus表中年龄大于18岁的学生姓名

mysql> select name from stus where age > 18;
Empty set (0.00 sec)

小于判断 <

? : 查找stus表中年龄小于18岁的学生姓名

mysql> select name from stus where age < 18;
+--------------+
| name         |
+--------------+
| 小猪佩奇     |
| 猪爸爸       |
| 猪妈妈       |
+--------------+

等于判断 =

? : 查找stus表中年龄等于18岁的学生姓名

mysql> select name from stus where age = 18;
+-----------+
| name      |
+-----------+
| 周棋洛    |
| 张郁苗    |
+-----------+

大于等于判断 >=

? : 查找stus表中年龄大于等于18岁的学生姓名

mysql> select name from stus where age >= 18;
+-----------+
| name      |
+-----------+
| 周棋洛    |
| 张郁苗    |
+-----------+

小于等于判断 <=

? : 查找stus表中年龄小于等于18岁的学生姓名

mysql> select name from stus where age <= 18;
+--------------+
| name         |
+--------------+
| 周棋洛       |
| 张郁苗       |
| 小猪佩奇     |
| 猪爸爸       |
| 猪妈妈       |
+--------------+

不等于判断 <> 或 !=

? : 查找stus表中年龄不等于18岁的学生姓名

mysql> select name from stus where age <> 18;
+--------------+
| name         |
+--------------+
| 小猪佩奇     |
| 猪爸爸       |
| 猪妈妈       |
+--------------+

mysql> select name from stus where age != 18;
+--------------+
| name         |
+--------------+
| 小猪佩奇     |
| 猪爸爸       |
| 猪妈妈       |
+--------------+

BETWEEN … AND …包含判断

? : 查找stus表中年龄在10~18岁的学生姓名

mysql> select name,age from stus where age between 8 and 20;
+-----------+------+
| name      | age  |
+-----------+------+
| 周棋洛    |   18 |
| 张郁苗    |   18 |
| 猪爸爸    |    8 |
+-----------+------+

IN(…)

? : 查找stus表中年龄为8岁或7岁的学生姓名

mysql> select name,age from stus where age in(7,8);
+-----------+------+
| name      | age  |
+-----------+------+
| 猪爸爸    |    8 |
| 猪妈妈    |    7 |
+-----------+------+

IS NULL

? : 查找stus表中名字为空的学生信息

mysql> select * from stus where name is null;
Empty set (0.02 sec)

IS NOT NULL

? : 查找stus表中名字不为空的学生信息

mysql> select * from stus where name is not null;
+----+---------+--------------+------+------------+---------+---------+
| id | gradeId | NAME         | age  | bir        | english | chinese |
+----+---------+--------------+------+------------+---------+---------+
|  1 |       1 | 周棋洛       |   18 | 2001-06-01 |      78 |    NULL |
|  2 |       2 | 张郁苗       |   18 | 2002-11-07 |     130 |     120 |
|  3 |       1 | 小猪佩奇     |    6 | 2015-06-10 |      34 |    NULL |
|  4 |       3 | 猪爸爸       |    8 | 2012-09-12 |      34 |      56 |
|  5 |       3 | 猪妈妈       |    7 | 2012-09-11 |      56 |      78 |
+----+---------+--------------+------+------------+---------+---------+

AND 或 &&

只有多个条件都成立才会返回真,否则就认为false

? : 查找stus表中年龄为18岁的周棋洛的学生信息

mysql> select * from stus where name = "周棋洛" AND age = "18";
+----+---------+-----------+------+------------+---------+---------+
| id | gradeId | NAME      | age  | bir        | english | chinese |
+----+---------+-----------+------+------------+---------+---------+
|  1 |       1 | 周棋洛    |   18 | 2001-06-01 |      78 |    NULL |
+----+---------+-----------+------+------------+---------+---------+

mysql> select * from stus where name = "周棋洛" && age = "18";
+----+---------+-----------+------+------------+---------+---------+
| id | gradeId | NAME      | age  | bir        | english | chinese |
+----+---------+-----------+------+------------+---------+---------+
|  1 |       1 | 周棋洛    |   18 | 2001-06-01 |      78 |    NULL |
+----+---------+-----------+------+------------+---------+---------+

OR 或 ||

只要有一个条件成立就返回真,都不成立返回false

? : 查找stus表中名字叫周棋洛或者叫张郁苗的学生信息

mysql> select * from stus where name = "周棋洛" OR name = "张郁苗";
+----+---------+-----------+------+------------+---------+---------+
| id | gradeId | NAME      | age  | bir        | english | chinese |
+----+---------+-----------+------+------------+---------+---------+
|  1 |       1 | 周棋洛    |   18 | 2001-06-01 |      78 |    NULL |
|  2 |       2 | 张郁苗    |   18 | 2002-11-07 |     130 |     120 |
+----+---------+-----------+------+------------+---------+---------+

mysql> select * from stus where name = "周棋洛" || name = "张郁苗";
+----+---------+-----------+------+------------+---------+---------+
| id | gradeId | NAME      | age  | bir        | english | chinese |
+----+---------+-----------+------+------------+---------+---------+
|  1 |       1 | 周棋洛    |   18 | 2001-06-01 |      78 |    NULL |
|  2 |       2 | 张郁苗    |   18 | 2002-11-07 |     130 |     120 |
+----+---------+-----------+------+------------+---------+---------+

模糊查询

糊查询 需要使用 like 占位符,几个案例学会模糊查询,下面是stus表的数据

mysql> select * from stus;
+----+---------+--------------+------+------------+---------+---------+
| id | gradeId | NAME         | age  | bir        | english | chinese |
+----+---------+--------------+------+------------+---------+---------+
|  1 |       1 | 周棋洛       |   18 | 2001-06-01 |      78 |    NULL |
|  2 |       2 | 张郁苗       |   18 | 2002-11-07 |     130 |     120 |
|  3 |       1 | 小猪佩奇     |    6 | 2015-06-10 |      34 |    NULL |
|  4 |       3 | 猪爸爸       |    8 | 2012-09-12 |      34 |      56 |
|  5 |       3 | 猪妈妈       |    7 | 2012-09-11 |      56 |      78 |
+----+---------+--------------+------+------------+---------+---------+

% 多个任意字符

1.查询 stus 表中 name 以猪开头的学生信息

mysql> select * from stus where name like "猪%";
+----+---------+-----------+------+------------+---------+---------+
| id | gradeId | NAME      | age  | bir        | english | chinese |
+----+---------+-----------+------+------------+---------+---------+
|  4 |       3 | 猪爸爸    |    8 | 2012-09-12 |      34 |      56 |
|  5 |       3 | 猪妈妈    |    7 | 2012-09-11 |      56 |      78 |
+----+---------+-----------+------+------------+---------+---------+

2.查询 stus 表中 name 以爸爸结尾的学生信息

mysql> select * from stus where name like '%爸爸';
+----+---------+-----------+------+------------+---------+---------+
| id | gradeId | NAME      | age  | bir        | english | chinese |
+----+---------+-----------+------+------------+---------+---------+
|  4 |       3 | 猪爸爸    |    8 | 2012-09-12 |      34 |      56 |
+----+---------+-----------+------+------------+---------+---------+

3.查询 stus 表中 name 中包含猪的学生信息

mysql> select * from stus where name like '%猪%';
+----+---------+--------------+------+------------+---------+---------+
| id | gradeId | NAME         | age  | bir        | english | chinese |
+----+---------+--------------+------+------------+---------+---------+
|  3 |       1 | 小猪佩奇     |    6 | 2015-06-10 |      34 |    NULL |
|  4 |       3 | 猪爸爸       |    8 | 2012-09-12 |      34 |      56 |
|  5 |       3 | 猪妈妈       |    7 | 2012-09-11 |      56 |      78 |
+----+---------+--------------+------+------------+---------+---------+

_ 单个任意字符

4.查询 stus 表中 name 以猪开头的且名字长度为2的学生信息

mysql> select * from stus where name like '猪_';
Empty set (0.00 sec)

5.查询 stus 表中 name 以猪开头的且名字长度为3的学生信息

mysql> select * from stus where name like '猪__';
+----+---------+-----------+------+------------+---------+---------+
| id | gradeId | NAME      | age  | bir        | english | chinese |
+----+---------+-----------+------+------------+---------+---------+
|  4 |       3 | 猪爸爸    |    8 | 2012-09-12 |      34 |      56 |
|  5 |       3 | 猪妈妈    |    7 | 2012-09-11 |      56 |      78 |
+----+---------+-----------+------+------------+---------+---------+

6.查询 stus 表中 name 以妈妈结尾的且名字长度为3的学生信息

mysql> select * from stus where name like '_妈妈';
+----+---------+-----------+------+------------+---------+---------+
| id | gradeId | NAME      | age  | bir        | english | chinese |
+----+---------+-----------+------+------------+---------+---------+
|  5 |       3 | 猪妈妈    |    7 | 2012-09-11 |      56 |      78 |
+----+---------+-----------+------+------------+---------+---------+

7.查询 stus 表中 name 以张开头以苗结尾的且名字长度为3的学生信息

mysql> select * from stus where name like '张_苗';
+----+---------+-----------+------+------------+---------+---------+
| id | gradeId | NAME      | age  | bir        | english | chinese |
+----+---------+-----------+------+------------+---------+---------+
|  2 |       2 | 张郁苗    |   18 | 2002-11-07 |     130 |     120 |
+----+---------+-----------+------+------------+---------+---------+

排序查询

关键字order by
关系型数据库设计理论认为,如果不明确规定排序顺序,则不应该假定检索出来的数据的顺序有意义,可以使用 order by 子句对查询结果进行排序,默认是升序,即从小到大 ASC,如果想要降序排序,则需要指定 DESC

ASC 升序

1. 查询stus表中名字字段,要求按照年龄的升序进行排序

mysql> select name from stus order by age;
+--------------+
| name         |
+--------------+
| 小猪佩奇     |
| 猪妈妈       |
| 猪爸爸       |
| 周棋洛       |
| 张郁苗       |
+--------------+

2. 查询stus表中名字,年龄字段,要求按照年龄大于7进行筛选之后按照年龄的升序进行排序

mysql> select name,age from stus where age > 7 order by age;
+-----------+------+
| name      | age  |
+-----------+------+
| 猪爸爸    |    8 |
| 周棋洛    |   18 |
| 张郁苗    |   18 |
+-----------+------+

DESC 降序

3. 查询stus表中名字字段,要求按照年龄的升序进行排序,如果年龄相同再按照英语成绩降序排列

mysql> select name from stus order by age,english desc;
+--------------+
| name         |
+--------------+
| 小猪佩奇     |
| 猪妈妈       |
| 猪爸爸       |
| 张郁苗       |
| 周棋洛       |
+--------------+

limit 选学

这个还没讲到,如果你不知道,可以跳过,后面会将到哦

4. 查询stus表中名字字段,要求按照年龄的升序进行排序,并通过limit返回一条数据,即年龄最小的

mysql> select name from stus order by age asc limit 1;
+--------------+
| name         |
+--------------+
| 小猪佩奇     |
+--------------+

哦,原来小猪佩奇是最小的

位置问题

在使用 order by 子句对检索出的数据进行排序时,应该保证它是位于 from,如果有条件,应位于where之后,如果使用 limit ,它必须位于 order by 之后,使用子句的次序不对将产生错误消息


聚合函数

MySQL 提供了5个聚合函数,聚合函数能够汇总数据,这些函数是高效设计的,它们返回结果一般比你自己在客户机应用程序中计算要快的多

AVG()

返回某列的平均值
查看stus表学生的平均英语成绩

mysql> select avg(english) as avgEnglish from stus;
+------------+
| avgEnglish |
+------------+
|    66.4000 |
+------------+

COUNT()

返回某列的行数
查看stus表一共有多少行

mysql> select count(*) as num from stus;
+-----+
| num |
+-----+
|   5 |
+-----+

MAX()

返回某列的最大值
查看stus表中英语成绩最高分

mysql> select max(english) as score from stus;
+-------+
| score |
+-------+
|   130 |
+-------+

MIN()

返回某列的最小值
查询stus表英语成绩的最低分

mysql> select min(english) as score from stus;
+-------+
| score |
+-------+
|    34 |
+-------+

还不错,没有0蛋

SUM()

返回某列之和
查询stus表中学生英语成绩的总分

mysql> select sum(english) as score from stus;
+-------+
| score |
+-------+
|   332 |
+-------+

分组查询

分组

对stus表的数据按照班级进行分组,并查看每个班都有多少人

mysql> select gradeId,count(*) as num from stus group by gradeId;
+---------+-----+
| gradeId | num |
+---------+-----+
|       1 |   2 |
|       2 |   1 |
|       3 |   2 |
+---------+-----+

对stus表的数据按照班级进行分组,查看每个班都有多少人以及班级英语平均分

mysql> select gradeId,avg(english)as english from stus group by gradeId;
+---------+----------+
| gradeId | english  |
+---------+----------+
|       1 |  56.0000 |
|       2 | 130.0000 |
|       3 |  45.0000 |
+---------+----------+

过滤

mysql> select gradeId,count(*) as num from stus group by gradeId having count(*) > 1;
+---------+-----+
| gradeId | num |
+---------+-----+
|       1 |   2 |
|       3 |   2 |
+---------+-----+

having和where的区别

  • 执行时间不一样:where是分组之前进行限定,不满足where条件,就不参与分组,而having是分组之后对结果进行过滤
  • 可判断的条件不一样:where 不能对聚合函数进行判断,having可以
  • where > 聚合函数 > having

select子句查询顺序

select

from

where 行级过滤

group by 分组

having 组级过滤

order by 输出排序顺序

limit 要检索的条目数


分页查询

关键字: limit
MySQL中使用limit来限制返回的条目数

一个参数,从开始处返回几条数据
select * from XXX limit 5;

mysql> select id,name,age from stus limit 3;
+----+--------------+------+
| id | name         | age  |
+----+--------------+------+
|  1 | 周棋洛       |   18 |
|  2 | 张郁苗       |   18 |
|  3 | 小猪佩奇     |    6 |
+----+--------------+------+

两个参数,注意检索出来的数据第一行为0而不是1,所以下面语句意思就是,从第一条数据算起,查询3条数据
select * from XXX limit 0,3;

mysql> select id,name,age from stus limit 0,2;
+----+-----------+------+
| id | name      | age  |
+----+-----------+------+
|  1 | 周棋洛    |   18 |
|  2 | 张郁苗    |   18 |
+----+-----------+------+

mysql> select id,name,age from stus limit 2,2;
+----+--------------+------+
| id | name         | age  |
+----+--------------+------+
|  3 | 小猪佩奇     |    6 |
|  4 | 猪爸爸       |    8 |
+----+--------------+------+

假如规定一页有10条记录
当查询第一页时就是 limit 0,10;
当查询第二页时就是 limit 10,10;
当查询第三页时就是 limit 20,10;
所以总结一下,查询第n页数据时,limit (n-1)*每页记录数,每页记录数;


多表查询

试着查询两张表

mysql> select * from stus,grade;
+----+---------+--------------+------+------------+---------+---------+----+--------------+
| id | gradeId | NAME         | age  | bir        | english | chinese | id | grade_name   |
+----+---------+--------------+------+------------+---------+---------+----+--------------+
|  1 |       1 | 周棋洛       |   18 | 2001-06-01 |      78 |    NULL |  1 | 高三一班     |
|  1 |       1 | 周棋洛       |   18 | 2001-06-01 |      78 |    NULL |  2 | 高三二班     |
|  1 |       1 | 周棋洛       |   18 | 2001-06-01 |      78 |    NULL |  3 | 高一二班     |
|  2 |       2 | 张郁苗       |   18 | 2002-11-07 |     130 |     120 |  1 | 高三一班     |
|  2 |       2 | 张郁苗       |   18 | 2002-11-07 |     130 |     120 |  2 | 高三二班     |
|  2 |       2 | 张郁苗       |   18 | 2002-11-07 |     130 |     120 |  3 | 高一二班     |
|  3 |       1 | 小猪佩奇     |    6 | 2015-06-10 |      34 |    NULL |  1 | 高三一班     |
|  3 |       1 | 小猪佩奇     |    6 | 2015-06-10 |      34 |    NULL |  2 | 高三二班     |
|  3 |       1 | 小猪佩奇     |    6 | 2015-06-10 |      34 |    NULL |  3 | 高一二班     |
|  4 |       3 | 猪爸爸       |    8 | 2012-09-12 |      34 |      56 |  1 | 高三一班     |
|  4 |       3 | 猪爸爸       |    8 | 2012-09-12 |      34 |      56 |  2 | 高三二班     |
|  4 |       3 | 猪爸爸       |    8 | 2012-09-12 |      34 |      56 |  3 | 高一二班     |
|  5 |       3 | 猪妈妈       |    7 | 2012-09-11 |      56 |      78 |  1 | 高三一班     |
|  5 |       3 | 猪妈妈       |    7 | 2012-09-11 |      56 |      78 |  2 | 高三二班     |
|  5 |       3 | 猪妈妈       |    7 | 2012-09-11 |      56 |      78 |  3 | 高一二班     |
+----+---------+--------------+------+------------+---------+---------+----+--------------+
15 rows in set (0.04 sec)

为啥返回给我15条结果啊❓

这样写的多表查询会有 笛卡尔积 :就是A,B两个集合,取 A,B所有的组合情况,比如A有3条记录,B有6条,查询就会有3*6=18条数据,显然不合符需求,所以要消除无效数据

内连接

关键字:INNER JOIN

连接结果仅包含符合连接条件的行组合起来作为结果集,参与连接的两个表都应该符合连接条件使用关键词:INNER JOIN 连接多张表

显示内连接

使用 INNER JOIN 关键字,条件使用 on 关键字

mysql> select grade_name, name from grade inner join stus on stus.gradeId = grade.id;
+--------------+--------------+
| grade_name   | name         |
+--------------+--------------+
| 高三一班     | 周棋洛       |
| 高三二班     | 张郁苗       |
| 高三一班     | 小猪佩奇     |
| 高一二班     | 猪爸爸       |
| 高一二班     | 猪妈妈       |
+--------------+--------------+

隐式内连接

内连接还有一种隐式的写法,即不需要显示的指定 INNER JOIN 关键字,需要注意,使用隐式内连接条件的关键字要使用 where 而不再是 on

mysql> select grade_name, name from grade,stus where stus.gradeId = grade.id;
+--------------+--------------+
| grade_name   | name         |
+--------------+--------------+
| 高三一班     | 周棋洛       |
| 高三二班     | 张郁苗       |
| 高三一班     | 小猪佩奇     |
| 高一二班     | 猪爸爸       |
| 高一二班     | 猪妈妈       |
+--------------+--------------+

一般我们常用直接使用where关键词查询连接条件这样更方便简单

外连接

关键字:OUTER JOIN

左外连接

LEFT JOIN

左(外)连接,左表的记录将会全部表示出来,而右表只会显示符合搜索条件的记录。右表记录不足的地方均为NULL

语法:

LEFT JOIN ON
LEFT OUTER JOIN ON

left joinleft outer join 的简写

mysql> select name,grade_name from stus s left outer join grade g on s.id = g.id;
+--------------+--------------+
| name         | grade_name   |
+--------------+--------------+
| 周棋洛       | 高三一班     |
| 张郁苗       | 高三二班     |
| 小猪佩奇     | 高一二班     |
| 猪爸爸       | NULL         |
| 猪妈妈       | NULL         |
+--------------+--------------+


mysql> select name,grade_name from stus s left join grade g on s.id = g.id;
+--------------+--------------+
| name         | grade_name   |
+--------------+--------------+
| 周棋洛       | 高三一班     |
| 张郁苗       | 高三二班     |
| 小猪佩奇     | 高一二班     |
| 猪爸爸       | NULL         |
| 猪妈妈       | NULL         |
+--------------+--------------+

右外连接

RIGHT JOIN

右(外)连接,右表的记录将会全部表示出来,而左表只会显示符合搜索条件的记录。左表记录不足的地方均为NULL

语法:

RIGHT JOIN ON
RIGHT OUTER JOIN ON

right joinright outer join 的简写

mysql> select name,grade_name from stus s right join grade g on s.id = g.id;
+--------------+--------------+
| name         | grade_name   |
+--------------+--------------+
| 周棋洛       | 高三一班     |
| 张郁苗       | 高三二班     |
| 小猪佩奇     | 高一二班     |
+--------------+--------------+

子查询

查询中嵌套查询,称嵌套查询为子查询

分类 (根据查询结果的不同,作用不同)

单行单列

作为条件值,使用 = != < > 等进行条件判断

语法:

select 字段列表 from 表 where 字段名 = (子查询);

例如:查询stus表名字叫周棋洛的在几班
1.首先,要在stus表中查询名字为周棋洛的 gradeId 是多少
2.根据查到的 gradeId 再去 grade 查询 相对应的 grade_name

mysql> select gradeId from stus where name = "周棋洛";
+---------+
| gradeId |
+---------+
|       1 |
+---------+

mysql> select grade_name from grade where id = 1;
+--------------+
| grade_name   |
+--------------+
| 高三一班     |
+--------------+

能不能把这两个sql语句合并为一个❓答案是毋庸置疑的,不可以,开玩笑的,哈哈,当然可以,要不也不说子查询了,开始合并

mysql> select grade_name from grade where id = (select gradeId from stus where name = "周棋洛");
+--------------+
| grade_name   |
+--------------+
| 高三一班     |
+--------------+

多行单列

作为条件值,用 in 等关键字进行条件判断

语法:

select 字段列表 from 表 where 字段名 in (子查询);

多行多列

作为虚拟表

select 字段列表 from (子查询) where t;





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