sql 高级语法

1. 并集与连接

1.1 union

查询多张表的结果，取并集，扩展行数。
如 select name from studentTable1 union select name from studentTable2

1.2 join

连接多张表中查询的结果，扩展列数。
SELECT * FROM Table1 JOIN Table2 ON Table1.ColumnA=Table2.columnA

left join，左连接，以左边表为基准；

right join，右连接，以右边表为基准。

两个表见图1-1，三条左连接语句见下，结果见图1-2。

图1-1 两张表t1与t2

/*1*/SELECT table1.id as id,table1.theValue as value1,table2.theValue as value2 FROM `t1` as table1 left JOIN t2 as table2 on table1.id=table2.id;

/*2*/SELECT table1.id as id,table1.theValue as value1,(case WHEN table2.theValue is null then 0 else table2.theValue end) as value2 FROM `t1` as table1 left JOIN t2 as table2 on table1.id=table2.id;

/*3*/select id from(SELECT table1.id as id,table1.theValue as value1,(case WHEN table2.theValue is null then 0 else table2.theValue end) as value2 FROM `t1` as table1 left JOIN t2 as table2 on table1.id=table2.id) AS A WHERE (value1-value2)/value1 > 0.1;

图1-2  三条语句的执行结果

1.3 join 与 where

可以在join前用where先筛出来一部分,join 后再筛选一部分, 如:

select * from Student where age between 13 and 16 inner join Score where mathScore > 95 on Student.id=Score.id where Student.height > 170

2.分组

Group By，分组。例 group by field1，依据field1字段的不同，将表划分成若干个“小区域”，然后针对这些“小区域”分别进行数据处理。

2.1 单个字段

group by 单个字段，见下。

图2-1 原始表

执行分组语句后：

select 类别, sum(数量) as 数量之和
from A
group by 类别

图2-2  group by 之后的结果

没有聚合函数的时候使用group by field1，效果就是field1字段相同的数据只保留一条。

2.2 多个字段

group by 多个字段，见下。

例group by field1,field2， 这两个字段对应一致的数据才会进入同一个分组。

3.having

在 SQL 中增加 HAVING 子句原因是，WHERE 关键字无法与聚合函数（如 min,sum等）一起使用。例子见图3-1.

图3-1 having用法

4. in 与 exists 的区别

in后面跟的是一个集合；exists 后面跟的是一个子查询，若查询到有行存在则输出父查询；

in 语句先执行子查询，将结果缓存；exists先执行父查询。

5.union与join

union
查询多张表的结果，取并集，扩展行数。
如 select name from studentTable1 union select name from studentTable2

join
连接多张表中查询的结果，扩展列数。
SELECT * FROM Table1 JOIN Table2 ON Table1.ColumnA=Table2.columnA

left join，左连接，以左边表为基准；

right join，右连接，以右边表为基准。

两个表见图5-1，三条左连接语句见下，结果见图4。

图5-1 两张表t1与t2

/*1*/SELECT table1.id as id,table1.theValue as value1,table2.theValue as value2 FROM `t1` as table1 left JOIN t2 as table2 on table1.id=table2.id;

/*2*/SELECT table1.id as id,table1.theValue as value1,(case WHEN table2.theValue is null then 0 else table2.theValue end) as value2 FROM `t1` as table1 left JOIN t2 as table2 on table1.id=table2.id;

/*3*/select id from(SELECT table1.id as id,table1.theValue as value1,(case WHEN table2.theValue is null then 0 else table2.theValue end) as value2 FROM `t1` as table1 left JOIN t2 as table2 on table1.id=table2.id) AS A WHERE (value1-value2)/value1 > 0.1;

图5-2  三条语句的执行结果

6.having

在 SQL 中增加 HAVING 子句原因是，WHERE 关键字无法与聚合函数（如 min,sum等）一起使用。例子见图3.

图6-1 having用法