mysql select 语法_MySQL
格式：select [选项子句] 字段表达式子句 [from子句] [where子句] [group by子句] [having子句] [order by子句] [limit子句];
提示：子句的顺序是固定的
select 选项子句 [ all | distinct ]
这个参数主要影响查询结果是否包含重复记录
All：代表保留所有的记录，这个默认选项，可以省略不写
Distinct：代表去除重复的记录
格式：select all|distinct 字段表达式 from...;
from子句
格式：from 数据表
FROM子句用于指定要查询使用的数据源
数据表可以有多个，中间用逗号分开，数据表也可以起别名
如果数据源是两个表时，结果会是一个交叉连接的方式，记录数相当于笛卡尔积
两个表的数据总和=表1的记录数*表2的记录数
写*时，字段名总数是两个表的字段数量的和
数据表也可以起别名，还可以加条件
WHERE子句
功能是进行数据的筛选过滤。
格式：where 条件表达式
条件表达式使用关系运算符和逻辑运算符
关系运算符
> 大于
>= 大于等于
!= 不等于
不等于
= 等于
逻辑运算符
&& and 与
|| or 或
! not 非
Xor 异或
运算符存在优先级的问题
在where子句中不能使用字段的别名
GROUP BY子句
主要用于分组。通常在字段表达式中使用分组函数。
格式：group by 字段1 [asc|desc], 字段2 [asc|desc], ……
排序不写默认是asc正序
使用count(*)和count(字段名)时，注意字段的值为null的问题
count(字段名)统计时，会把值为null的记录忽略掉！
HAVING子句
它用于对查询出来的结果进行再次的过滤。
比较显著的就是如对分组查询出来的结果再次使用条件。
省略where和group by子句时，不等于having就是where
不能把where放到having的位置
ORDER BY子句
主要用于对数据进行排序。
格式：order by 字段1 [asc|desc], 字段2 [asc|desc], ……
Asc代表正序，它是默认选项，不写就是asc
Desc代表倒序
当用一个字段做排序时，如果出现针对排序字段相同的记录时，会隐含去使用主键进行第二排序
LIMIT子句
这个子句是做分页的。
格式：limit 起始记录位置, 记录数
注意：如果省略起始记录位置，只给出记录数，表示从第一条数据开始取内容
Limit 10 相当于 limit 0, 10
提示：记录的位置是从0开始计算的
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