Postgresql 简明教程
PostgreSQL - GROUP BY
PostgreSQL GROUP BY 子句与 SELECT 语句结合使用,用于将表中具有相同数据的行组合在一起。这用于消除输出中的冗余和/或计算适用于这些组的聚合。
GROUP BY 子句在 SELECT 语句中紧跟在 WHERE 子句之后,并且出现在 ORDER BY 子句之前。
Syntax
GROUP BY 子句的基本语法如下。GROUP BY 子句必须紧跟在 WHERE 子句中的条件之后,并且如果使用 ORDER BY 子句,必须出现在 ORDER BY 子句之前。
SELECT column-list
FROM table_name
WHERE [ conditions ]
GROUP BY column1, column2....columnN
ORDER BY column1, column2....columnN
您可以在 GROUP BY 子句中使用多个列。确保您用来进行分组的列,此列在 column-list 中可用。
Example
考虑 COMPANY 表具有以下记录:
# 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)
如果您想了解每个客户的总薪水,则 GROUP BY 查询如下所示 −
testdb=# SELECT NAME, SUM(SALARY) FROM COMPANY GROUP BY NAME;
这将产生以下结果 -
name | sum
-------+-------
Teddy | 20000
Paul | 20000
Mark | 65000
David | 85000
Allen | 15000
Kim | 45000
James | 10000
(7 rows)
现在,让我们使用以下 INSERT 语句在 COMPANY 表中创建三条更多记录 −
INSERT INTO COMPANY VALUES (8, 'Paul', 24, 'Houston', 20000.00);
INSERT INTO COMPANY VALUES (9, 'James', 44, 'Norway', 5000.00);
INSERT INTO COMPANY VALUES (10, 'James', 45, 'Texas', 5000.00);
现在,我们的表有以下具有重复名称的记录 −
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
8 | Paul | 24 | Houston | 20000
9 | James | 44 | Norway | 5000
10 | James | 45 | Texas | 5000
(10 rows)
同样,让我们使用相同的语句按 NAME 列对所有记录进行分组,如下所示 −
testdb=# SELECT NAME, SUM(SALARY) FROM COMPANY GROUP BY NAME ORDER BY NAME;
这将产生以下结果 -
name | sum
-------+-------
Allen | 15000
David | 85000
James | 20000
Kim | 45000
Mark | 65000
Paul | 40000
Teddy | 20000
(7 rows)
让我们将 ORDER BY 子句与 GROUP BY 子句结合使用,如下所示 −
testdb=# SELECT NAME, SUM(SALARY)
FROM COMPANY GROUP BY NAME ORDER BY NAME DESC;
这将产生以下结果 -
name | sum
-------+-------
Teddy | 20000
Paul | 40000
Mark | 65000
Kim | 45000
James | 20000
David | 85000
Allen | 15000
(7 rows)