无涯教程-PostgreSQL - Group By语句_pgsql group by

PostgreSQL GROUP BY 子句与SELECT语句配合使用，将表中具有相同数据的行组合在一起，GROUP BY子句在SELECT语句中的WHERE子句之后，在ORDER BY子句之前。

Group By - 语法

GROUP BY子句的基本语法如下，GROUP BY子句必须遵循WHERE子句中的条件，并且如果使用ORDER BY子句，则必须在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，确保要用于分组的任何列，该列都应在column-list中可用。

Group By - 示例

考虑表 COMPANY 具有以下记录:

# select * from COMPANY;
 id | name  | age | address   | salary
----+-------+-----+-----------+--------
  1 | Paul  |  32 | California|  20000
  2 | Learnfk |  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)

如果您想知道每个客户的工资总额，那么GROUP BY查询将如下所示-

testdb=# SELECT NAME, SUM(SALARY) FROM COMPANY GROUP BY NAME;

这将产生以下输出-

  name  |  sum
 -------+-------
  Teddy | 20000
  Paul  | 20000
  Mark  | 65000
  David | 85000
  Learnfk | 15000
  Kim   | 45000
  James | 10000
(7 rows)

现在，让无涯教程使用以下INSERT语句在COMPANY表中再创建三个记录-

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，无涯教程的表具有以下重复名称的记录-

  id | name  | age | address      | salary
 ----+-------+-----+--------------+--------
   1 | Paul  |  32 | California   |  20000
   2 | Learnfk |  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)

再次，让无涯教程使用相同的语句使用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;

这将产生以下输出-

 name  |  sum
-------+-------
 Learnfk | 15000
 David | 85000
 James | 20000
 Kim   | 45000
 Mark  | 65000
 Paul  | 40000
 Teddy | 20000
(7 rows)

让无涯教程如下使用ORDER BY子句和GROUP BY子句-

testdb=#  SELECT NAME, SUM(SALARY)
         FROM COMPANY GROUP BY NAME ORDER BY NAME DESC;

这将产生以下输出-

 name  |  sum
-------+-------
 Teddy | 20000
 Paul  | 40000
 Mark  | 65000
 Kim   | 45000
 James | 20000
 David | 85000
 Learnfk | 15000
(7 rows)

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