在PostgreSQL中,WITH查询提供了一种编写辅助语句以便在更大的查询中使用的方法.它有助于将复杂的大型查询分解为易于阅读的简单形式.这些语句通常称为公用表表达式或CTE,可以被认为是定义仅存在于一个查询的临时表.
WITH查询是CTE查询,在子查询时特别有用被执行多次.它同样有助于代替临时表.它计算聚合一次,并允许我们在查询中通过其名称(可能是多次)引用它.
必须在查询中使用WITH子句之前定义它.
WITH查询的基本语法如下 :
WITH name_for_summary_data AS ( SELECT Statement) SELECT columns FROM name_for_summary_data WHERE conditions <=> ( SELECT column FROM name_for_summary_data) [ORDER BY columns]
其中 name_for_summary_data 是赋予WITH子句的名称. name_for_summary_data可以与现有表名相同,并且优先.
您可以在WITH中使用数据修改语句(INSERT,UPDATE或DELETE).这允许您在同一查询中执行多个不同的操作.
递归WITH或Hierarchical查询,是CTE可以引用的CTE形式对于它自身,即一个WITH查询可以引用它自己的输出,因此名称是递归的.
考虑表公司,记录如下 :
testdb# select * from COMPANY; id | name | age | address | salary ----+-------+-----+-----------+-------- 1 | Paul | 32 | California| 20000 2 | Allen | 25 | Texas | 15000 3 | Teddy | 23 | Norway | 20000 4 | Mark | 25 | Rich-Mond | 65000 5 | David | 27 | Texas | 85000 6 | Kim | 22 | South-Hall| 45000 7 | James | 24 | Houston | 10000 (7 rows)
现在,让我们使用WITH子句编写一个查询来从上表中选择记录,如下所示;
With CTE AS (Select ID , NAME , AGE , ADDRESS , SALARY FROM COMPANY ) Select * From CTE;
上面给出的PostgreSQL语句将产生以下结果 :
id | name | age | address | salary ----+-------+-----+-----------+-------- 1 | Paul | 32 | California| 20000 2 | Allen | 25 | Texas | 15000 3 | Teddy | 23 | Norway | 20000 4 | Mark | 25 | Rich-Mond | 65000 5 | David | 27 | Texas | 85000 6 | Kim | 22 | South-Hall| 45000 7 | James | 24 | Houston | 10000 (7 rows)
现在,让我们使用RECURSIVE关键字和WITH子句编写查询,以查找总和工资低于20000,如下 :
WITH RECURSIVE t(n) AS ( VALUES (0) UNION ALL SELECT SALARY FROM COMPANY WHERE SALARY < 20000 ) SELECT sum(n) FROM t;
上面给出的PostgreSQL语句将产生以下结果 :
sum ------- 25000 (1 row)
让我们用一个查询来编写一个查询数据修改语句以及WITH子句,如下所示.
首先,创建一个类似于表COMPANY的表COMPANY1.示例中的查询有效地将行从COMPANY移动到COMPANY1. WITHETE中的DELETE删除COMPANY中的指定行,通过其RETURNING子句返回其内容;然后主查询读取该输出并将其插入到COMPANY1 TABLE :
CREATE TABLE COMPANY1( ID INT PRIMARY KEY NOT NULL, NAME TEXT NOT NULL, AGE INT NOT NULL, ADDRESS CHAR(50), SALARY REAL ); WITH moved_rows AS ( DELETE FROM COMPANY WHERE SALARY >= 30000 RETURNING * ) INSERT INTO COMPANY1 (SELECT * FROM moved_rows);
上面给出的PostgreSQL语句将产生以下结果 :
INSERT 0 3
现在,表COMPANY和COMPANY1中的记录如下 :
testdb=# SELECT * FROM COMPANY; id | name | age | address | salary ----+-------+-----+------------+-------- 1 | Paul | 32 | California | 20000 2 | Allen | 25 | Texas | 15000 3 | Teddy | 23 | Norway | 20000 7 | James | 24 | Houston | 10000 (4 rows) testdb=# SELECT * FROM COMPANY1; id | name | age | address | salary ----+-------+-----+-------------+-------- 4 | Mark | 25 | Rich-Mond | 65000 5 | David | 27 | Texas | 85000 6 | Kim | 22 | South-Hall | 45000 (3 rows)