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)
原创文章，作者：Maggie-Hunter，如若转载，请注明出处：https://blog.ytso.com/4856.html