功能介紹

CTE可以被認為是在單個DML語句的執行范圍內定義的臨時結果集。CTE類似于派生表，它不作為對象存儲，并且僅在查詢期間持續。開發過程中結合CTE，可以提高SQL語句可讀性，便于輕松維護復雜查詢。

命令格式

with 
     <cte_name> as
    (
        <cte_query>
    )
    [,<cte_name2>  as 
     (
     <cte_query2>
     )
    ,……]

• cte_name：必填。CTE的名稱，不能與當前with子句中的其他CTE的名稱相同。查詢中任何使用到cte_name標識符的地方，均指CTE。

• cte_query：必填。一個select語句。select的結果集用于填充CTE。

使用示例

假設現有如下代碼：

insert overwrite table srcp partition (p='abc')
select * from (
    select a.key, b.value
    from (
        select * from src where key is not null    ) a
    join (
        select * from src2 where value > 0    ) b
    on a.key = b.key
) c
union all
select * from (
    select a.key, b.value
    from (
        select * from src where key is not null    ) a
    left outer join (
        select * from src3 where value > 0    ) b
    on a.key = b.key and b.key is not null
)d;

頂層的union兩側各為一個join，join的左表是相同的查詢語句。通過寫子查詢的方式，只能重復這段代碼。

with 
  a as (select * from src where key is not null),
  b as (select * from src2 where value > 0),
  c as (select * from src3 where value > 0),
  d as (select a.key, b.value from a join b on a.key=b.key),
  e as (select a.key,c.value from a left outer join c on a.key=c.key and c.key is not null)
insert overwrite table srcp partition (p='abc')
select * from d union all select * from e;

重寫后，a對應的子查詢只需寫一次，便可在后面進行重用。您可以在CTE的with子句中指定多個子查詢，像使用變量一樣在整個語句中反復重用。除重用外，不必反復嵌套。