union可以合并两个以上 select语句查询出来的表,并消除表中的重复行。
其中,select语句需要拥有相同数量和相同数据类型的列。
select ProID,ProName from T_Province
select CityID,CityName from T_City where ProID = ( select ProID from T_Province where ProName="湖南省" );
select ProID,ProName from T_Province union select CityID,CityName from T_City where ProID = ( select ProID from T_Province where ProName="湖南省" );
这样就得到两个查询结果的并集了。
UNION 合并后的集合中的列名总是等于 UNION 中第一个 SELECT 语句中的列名。
select ProID,ProName from T_Province union all select CityID,CityName from T_City where ProID = ( select ProID from T_Province where ProName="湖南省" );
当使用union all,不会消除重复行。
不用联合查询:
select count(CityID) from T_City where ProID = (select ProID from T_Province where ProName="湖北省")
通过ProID将两张表连接在一起
select ProName,CityName from( T_City join T_Province on T_City.ProID = T_Province.ProID ) where ProName="湖北省"
select T_City.ProID,ProName,count(CityID) as cc from( T_City join T_Province on T_City.ProID = T_Province.ProID ) group by T_City.ProID order by cc desc;
什么的select语句中要输出的ProID应该是T_City和T_Province中的一个,不然就会报错。
两个表之间需要有共同的(列名不一定相同)“语言”才能join。
可以给表起个别名,将T_City表的别名设为tc,将T_Province的别名设为tp。
select tc.ProID,ProName,count(CityID) as cc from( T_City tc join T_Province tp on T_City.ProID = T_Province.ProID ) group by tc.ProID order by cc desc;
select CityName,count(DisName) disCount from ( T_City tc join T_District td on tc.CityID = td.CityID ) group by CityName having disCount > 20;
select tp.ProName,tcd.CityName,tcd.ci from ( select ProID,CityName,count(ID) ci from(T_City tc join T_District td on tc.CityID = td.CityID) group by tc.CityID order by ci desc limit 3 )tcd join T_Province tp on tcd.ProID = tp.ProID;
内连接是基于左右两表公共的部分
左连接是基于左右两表公共的部分加上左表特有的部分
右连接是基于左右两表公共的部分加上右表特有的部分
select * from( T_Province tp join T_City tc on tp.ProID = tc.ProID );
select * from( T_Province tp left join T_City tc on tp.ProID = tc.ProID );
select * from( T_Province tp right join T_City tc on tp.ProID = tc.ProID );