With查询语句已with开头，相当于在查询之前先构建一个临时表，被指定的查询结果存与临时表中，之后便可多次使用它做进一步的分析和处理。
语法：
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with _tempTable as (select * from table )
select * from _tempTable例子：
with _tempStudent as(
select * from t_student t where class = '初二3班'
)
select sex,count(1) nums from _tempStudent where sex = '男' and height > '170'
union all
select sex,count(1) nums from _tempStudent where sex = '女' and height > '160'多个with as 用法 每个临时存量直接用 "," 隔开
with t1 as (
select * from student where name in('张三','李四')
),t2 as (
select * from student where name in('王五')
)
select * from t1
union
select * from t2如果with as 有嵌套的情况， 多个with as，后面的as内部可以直接调用先声明的临时对象
with t1 as (
select * from student where name in('张三','李四')
),t2 as (
select * from t1 where name in('王五')
)
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