postgresql----排序ORDER BY，分组GROUP BY，分页OFFSET&&LIMIT

一.GROUP BY

使用GROUP BY分组查询在SELECT子句中只能出现分组字段和聚合函数，HAVING子句相当于WHERE，使用条件过滤数据。

示例1.以a,b分组查询tbl_insert表，且a大于5的行。

test=# select a,b,count(*) from tbl_insert group by a,b having a > 5;
 a | b | count 
---+---+-------
 7 | 7 |     4
 8 | 8 |     2
 6 | 6 |     3
(3 rows)

test=# select a,b,count(*) from tbl_insert where a > 5 group by a,b;
 a | b | count 
---+---+-------
 7 | 7 |     4
 8 | 8 |     2
 6 | 6 |     3
(3 rows)

示例2.以a,b分组查询tbl_insert表，且a,b组合相同的行数大于2的a,b组合

test=# select a,b,count(*) from tbl_insert group by a,b having count(*) > 2;
 a | b | count 
---+---+-------
 7 | 7 |     4
 6 | 6 |     3
(2 rows)

示例3.以a,b分组查询tbl_insert表，如果SELECT子句中包含其他字段，则ERROR。

test=# select a,b,c from tbl_insert group by a,b;
ERROR:  column "tbl_insert.c" must appear in the GROUP BY clause or be used in an aggregate function
LINE 1: select a,b,c from tbl_insert group by a,b;

二.ORDER BY

使用ORDER BY对结果集进行排序，默认使用ASC升序排列，可指定DESC降序排列。

示例1.查询tbl_insert表按照a升序,c降序排列

test=# select * from tbl_insert where a > 6 order by a ,c desc;
 a | b |   c   
---+---+-------
 7 | 7 | abc
 7 | 7 | aBC
 7 | 7 | ABc
 7 | 7 | 3%1
 8 | 8 | 3_%_1
 8 | 8 | 3%_1
(6 rows)

示例2.以字段a,b分组查询tbl_insert表，并按照各分组中行数降序排列。

test=# select a,b,count(*) from tbl_insert group by a,b order by count(*) desc;
 a | b | count 
---+---+-------
 7 | 7 |     4
 6 | 6 |     3
 8 | 8 |     2
 4 | 4 |     1
 5 | 5 |     1
 1 | 1 |     1
 3 | 3 |     1
 2 | 2 |     1
(8 rows)

三.LIMIT

使用LIMIT可以限制结果集的行数。

示例1.查询tbl_insert表，并以a降序排列，只输出前5行数据。

test=# select * from tbl_insert order by a desc limit 5;
 a | b |   c   
---+---+-------
 8 | 8 | 3_%_1
 8 | 8 | 3%_1
 7 | 7 | 3%1
 7 | 7 | ABc
 7 | 7 | abc
(5 rows)

四.OFFSET

使用OFFSET可以忽略前面的N行，与LIMIT结合使用可以实现分页功能。

示例1.查询tbl_insert表，并以a升序排列，前10行忽略不输出。

test=# select * from tbl_insert order by a offset 10;
 a | b |   c   
---+---+-------
 7 | 7 | ABc
 7 | 7 | aBC
 8 | 8 | 3%_1
 8 | 8 | 3_%_1
(4 rows)

示例2.查询tbl_insert表，并以a升序排列，实现分页功能，每页5行数据。

test=# select * from tbl_insert order by a offset 0 limit 5;
 a | b | c  
---+---+----
 1 | 1 | 11
 2 | 2 | 22
 3 | 3 | 33
 4 | 4 | 44
 5 | 5 | 51
(5 rows)

test=# select * from tbl_insert order by a offset 5 limit 5;
 a | b |  c  
---+---+-----
 6 | 6 | 661
 6 | 6 | 1
 6 | 6 | 61
 7 | 7 | 3%1
 7 | 7 | abc
(5 rows)

test=# select * from tbl_insert order by a offset 10 limit 5;
 a | b |   c   
---+---+-------
 7 | 7 | ABc
 7 | 7 | aBC
 8 | 8 | 3%_1
 8 | 8 | 3_%_1
(4 rows)