sql2
select count(列名) from 表名 #查询列中不同字段数distinct 去重
where 条件查询 在group by 之前
hving 在 group by 之后
如果在from 后面的表必须得起别名
在 where 后面的表不需要
SQL 语句的书写顺序如下：
SELECT -> FROM -> WHERE -> GROUP BY -> HAVING -> ORDER BY
SQL 语句的执行顺序如下：
FROM -> WHERE -> GROUP BY -> HAVING -> SELECT -> ORDER BY
select语句的处理顺序
[*]FROM
[*]ON
[*]JOIN
[*]WHERE
[*]GROUP BY
[*]WITH CUBE 或 WITH ROLLUP
[*]HAVING
[*]SELECT
[*]DISTINCT
[*]ORDER BY
[*]TOP
mod(id, 2) = 1#条件判断取模运算<br>order byrating Desc#按rating值排列 desc 反向<br><br><br><br><br>update Salary SET sex = if(sex = 'm','f','m') #1. update 表名 set 条件判断语句 # 2. if判断 if(列对象 = 值,case1,case2) 若为真,则执行case1,否则执行case2
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