1. 基本语法：
SELECT {* | <字段列名>} [ FROM <表 1>, <表 2>… [WHERE <表达式> [GROUP BY <group by definition> [HAVING <expression> [{<operator> <expression>}…]] [ORDER BY <order by definition>] [LIMIT[<offset>,] <row count>] ]
1.1 全部查询
SELECT * FROM < 表名 >;
1.2 选择指定字段
SELECT <字段名1>,<字段名2>,…,<字段名n> FROM <表名>;
1.3 过滤字段中的重复值: DISTINCT
SELECT DISTINCT <字段名> FROM <表名>;
1.4 别名 :AS
给很长的表或者字段可以起个别名，用别名进行查询
mysql> select a.id,a.name from user as a; +----+------+ | id | name | +----+------+ | 1 | Wang | +----+------+
mysql> select id as userID from user;
+--------+
| userID |
+--------+
| 1 |
+--------+
1.5 限制查询结果的记录数量：LIMIT
<LIMIT> [<位置偏移量>,] <行数>
“位置偏移量”指从哪一行开始显示，是一个可选参数，如果不指定“位置偏移量”，将会从表中的第一条记录开始（第一条记录的位置偏移量是 0，第二条记录的位置偏移量是 1，以此类推）；
第二个参数“行数”指示返回的记录条数。
# 查询从第3+1条记录开始的4条数据。
mysql> SELECT * FROM <表名> LIMIT 3,4;
1.6 排序查询：ORDER BY
ORDER BY {<列名> | <表达式> | <位置>} [ASC|DESC]
ASC 升序
DESC 降序
e.g.
从user中查询所有字段，先按照 balance 字段进行降序排列，再按照 debt 字段升序排序
mysql> select * from user order by balance desc,debt asc; +----+------+----------+---------+------+ | id | name | password | balance | debt | +----+------+----------+---------+------+ | 1 | Wang | 1234 | 15000 | 0 | | 2 | song | 1234 | 3000 | 100 | | 3 | li | 1234 | 2000 | 0 | | 4 | zhao | NULL | NULL | NULL | | 5 | qian | NULL | NULL | NULL | | 6 | sun | NULL | NULL | NULL | | 9 | NULL | NULL | NULL | 70 | | 8 | NULL | NULL | NULL | 80 | | 7 | Zhou | NULL | NULL | 90 | +----+------+----------+---------+------+
1.7 条件查询： WHERE
WHERE <查询条件> {<判定运算1>，<判定运算2>，…}
判断条件：
1. >,<,<=,>=,=,<>,!=,<=> # 注：<=> 判断两个数据是否相等，只是可以和 NULL 进行比较，其他方面和 = 没啥差别。
2. [NOT]LIKE
3. IS [NOT] NULL
4. [NOT] BETWEEN ... AND ...
5. [NOT][REGEXP|RLIKE]
单一查询：
mysql> select balance from user where name = 'Wang'; +---------+ | balance | +---------+ | 15000 | +---------+
多条件查询：
mysql> select name,balance from user where balance>=3000 and balance<16000; +------+---------+ | name | balance | +------+---------+ | Wang | 15000 | | song | 3000 | +------+---------+
1.8 LIKE 模糊查询
<表达式1> [NOT] LIKE <表达式2>
通配符：
1. 百分号：% ： 可以匹配 0 个或多个字符，不匹配 空值。
2. 下划线：_ ： 只匹配一个字符。
mysql> select * from user; +----+------+----------+---------+------+ | id | name | password | balance | debt | +----+------+----------+---------+------+ | 1 | Wang | 1234 | 15000 | 0 | | 2 | song | 1234 | 3000 | 100 | | 3 | li | 1234 | 2000 | 0 | | 4 | zhao | NULL | NULL | NULL | | 5 | qian | NULL | NULL | NULL | | 6 | sun | NULL | NULL | NULL | | 7 | Zhou | NULL | NULL | 90 | | 8 | NULL | NULL | NULL | 80 | | 9 | NULL | NULL | NULL | 70 | +----+------+----------+---------+------+ mysql> select name,debt from user where name like '%ng'; +------+------+ | name | debt | +------+------+ | Wang | 0 | | song | 100 | +------+------+
1.9 查询日期
select * from <table> where date<'2019-01-20'; select * from <table> where date between '2019-01-20' and '2019-10-01';
2.0 内连接查询
mysql> select * from grade; +------+--------+ | gid | record | +------+--------+ | 1 | 100 | | 2 | 200 | | 3 | 400 | +------+--------+ mysql> select * from user; +----+------+----------+---------+------+ | id | name | password | balance | debt | +----+------+----------+---------+------+ | 1 | Wang | 1234 | 15000 | 0 | | 2 | song | 1234 | 3000 | 100 | | 3 | li | 1234 | 2000 | 0 | | 4 | zhao | NULL | NULL | NULL | | 5 | qian | NULL | NULL | NULL | | 6 | sun | NULL | NULL | NULL | | 7 | Zhou | NULL | NULL | 90 | | 8 | NULL | NULL | NULL | 80 | | 9 | NULL | NULL | NULL | 70 | +----+------+----------+---------+------+
# 内连接查询 mysql> select * from user inner join grade where user.id=grade.gid; +----+------+----------+---------+------+------+--------+ | id | name | password | balance | debt | gid | record | +----+------+----------+---------+------+------+--------+ | 1 | Wang | 1234 | 15000 | 0 | 1 | 100 | | 2 | song | 1234 | 3000 | 100 | 2 | 200 | | 3 | li | 1234 | 2000 | 0 | 3 | 400 | +----+------+----------+---------+------+------+--------+
# 不写inner join，默认也是内连接 mysql> select * from user, grade where user.id=grade.gid; +----+------+----------+---------+------+------+--------+ | id | name | password | balance | debt | gid | record | +----+------+----------+---------+------+------+--------+ | 1 | Wang | 1234 | 15000 | 0 | 1 | 100 | | 2 | song | 1234 | 3000 | 100 | 2 | 200 | | 3 | li | 1234 | 2000 | 0 | 3 | 400 | +----+------+----------+---------+------+------+--------+
# 指定查询哪些字段 mysql> select user.id,user.name,user.balance,grade.gid from user, grade where user.id=grade.gid; +----+------+---------+------+ | id | name | balance | gid | +----+------+---------+------+ | 1 | Wang | 15000 | 1 | | 2 | song | 3000 | 2 | | 3 | li | 2000 | 3 | +----+------+---------+------+
2.1 外连接
左外连接：
基于左边的表，将右边表中的字段合并过来
mysql> select id,name,record from user left join grade on user.id = grade.gid; +----+------+--------+ | id | name | record | +----+------+--------+ | 1 | Wang | 100 | | 2 | song | 200 | | 3 | li | 400 | | 4 | zhao | NULL | | 5 | qian | NULL | | 6 | sun | NULL | | 7 | Zhou | NULL | | 8 | NULL | NULL | | 9 | NULL | NULL | +----+------+--------+
右外连接：
mysql> select id,name,record from user right join grade on user.id = grade.gid; +------+------+--------+ | id | name | record | +------+------+--------+ | 1 | Wang | 100 | | 2 | song | 200 | | 3 | li | 400 | +------+------+--------+
2.2 子查询
1. IN
<表达式> [NOT] IN <子查询>
此处的子查询只能返回一行数据
2. 比较运算符
<表达式> {= | < | > | >= | <= | <=> | < > | != } { ALL | SOME | ANY} <子查询>
3. EXISTS
判断子查询是否存在，如果存在，True，否则False；
IN：
mysql> select * from grade; +------+--------+ | gid | record | +------+--------+ | 1 | 100 | | 2 | 200 | | 3 | 400 | +------+--------+ mysql> select * from user; +----+------+----------+---------+------+ | id | name | password | balance | debt | +----+------+----------+---------+------+ | 1 | Wang | 1234 | 15000 | 0 | | 2 | song | 1234 | 3000 | 100 | | 3 | li | 1234 | 2000 | 0 | | 4 | zhao | NULL | NULL | NULL | | 5 | qian | NULL | NULL | NULL | | 6 | sun | NULL | NULL | NULL | | 7 | Zhou | NULL | NULL | 90 | | 8 | NULL | NULL | NULL | 80 | | 9 | NULL | NULL | NULL | 70 | +----+------+----------+---------+------+ mysql> select * from user where id in (select gid from grade where gid=2); +----+------+----------+---------+------+ | id | name | password | balance | debt | +----+------+----------+---------+------+ | 2 | song | 1234 | 3000 | 100 | +----+------+----------+---------+------+
EXISTS：
mysql> select * from user where exists (select * from grade where gid=1000); Empty set (0.00 sec) mysql> select * from user where exists (select * from grade where gid=1); +----+------+----------+---------+------+ | id | name | password | balance | debt | +----+------+----------+---------+------+ | 1 | Wang | 1234 | 15000 | 0 | | 2 | song | 1234 | 3000 | 100 | | 3 | li | 1234 | 2000 | 0 | | 4 | zhao | NULL | NULL | NULL | | 5 | qian | NULL | NULL | NULL | | 6 | sun | NULL | NULL | NULL | | 7 | Zhou | NULL | NULL | 90 | | 8 | NULL | NULL | NULL | 80 | | 9 | NULL | NULL | NULL | 70 | +----+------+----------+---------+------+
2.3 分组查询
分组查询查到是每个组，相同的数据会合并到一个组里，是看不到的。可以使用聚合函数对每个组的数据进行求和、统计数量等。
mysql> select * from user; +----+------+----------+---------+------+ | id | name | password | balance | debt | +----+------+----------+---------+------+ | 1 | Wang | 1234 | 15000 | 0 | | 2 | song | 1234 | 3000 | 100 | | 3 | li | 1234 | 2000 | 0 | | 4 | zhao | NULL | NULL | NULL | | 5 | qian | NULL | NULL | NULL | | 6 | sun | NULL | NULL | NULL | | 7 | Zhou | NULL | NULL | 90 | | 10 | Wang | NULL | 15000 | NULL | +----+------+----------+---------+------+ # 按照名字分组，查询每个分组的id，name，balance的总和。sum，聚合函数之一。 mysql> select id,name,sum(balance) from user group by name; +----+------+--------------+ | id | name | sum(balance) | +----+------+--------------+ | 1 | Wang | 30000 | | 2 | song | 3000 | | 3 | li | 2000 | | 4 | zhao | NULL | | 5 | qian | NULL | | 6 | sun | NULL | | 7 | Zhou | NULL | +----+------+--------------+
HAVING
having，给分组添加过滤，筛选符合条件的分组
mysql> select count(name) ,name from user group by name having count(name)>1; +-------------+------+ | count(name) | name | +-------------+------+ | 2 | Wang | +-------------+------+
2.4 正则表达式查询：REGEXP
匹配模式
^ 匹配开始字符 '^b'匹配以b开头的字符串
$ 匹配结束字符 'b$'匹配以b结束的字符串
. 匹配任意单个字符
* 匹配前面的字符0次或多次 a*b 匹配 b、ab、aaaaaaaab
+ 匹配前面的字符1次或多次 a+b 匹配 ab、aaaaaab
<字符> 匹配包含的字符 <fa> 匹配 afa,fa,bbbfaccc
[] 匹配内部的任意一个字符 [xyz] 匹配 x 或 y 或 z
[^] 匹配不在括号内的字符 [^1-9] 不匹配 1-9
{n,} 匹配前一个字符最少n次
{n,m} 匹配前一个字符最少n次，最多m次。
e.g.
mysql> select * from user where name regexp '^W'; +----+------+----------+---------+------+ | id | name | password | balance | debt | +----+------+----------+---------+------+ | 1 | Wang | 1234 | 15000 | 0 | | 10 | Wang | NULL | 15000 | NULL | +----+------+----------+---------+------+ 2 rows in set (0.02 sec)