表的join操作:left、right、outer、inner
2016-09-07 10:47
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尽量不用where in …的句子,效率不高
首先join操作是为了扩展列的数目。
select * from A inner join B on A.field1=B.field2 或
select * from A,B where A.field1=B.field2
连接的记录数与A表的记录数同 ,如果B中没有匹配的,就以null值填充。
A right join B 的连接的记录数与B表的记录数同
写成left join、right join默认是外连接,即left (outer) join…
注意:没有匹配用null填充时,排序时如果用到该字段会影响排序。
可以这样写:
首先join操作是为了扩展列的数目。
内连接
内连接只查询匹配的记录,匹配不上的记录无法查询。select * from A inner join B on A.field1=B.field2 或
select * from A,B where A.field1=B.field2
外连接
select * from A left join B on A.field1=B.field2连接的记录数与A表的记录数同 ,如果B中没有匹配的,就以null值填充。
A right join B 的连接的记录数与B表的记录数同
写成left join、right join默认是外连接,即left (outer) join…
注意:没有匹配用null填充时,排序时如果用到该字段会影响排序。
可以这样写:
SELECT (case when sum(b.CLICKS) is null then 0 else sum(b.CLICKS) end) count FROM 。。。。 ORDER BY count DESC
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