table_AID Name Quantity
1 apple 5
2 banana 1
3 orange 1table_BID Name Quantity
1 apple 2
2 banana 1
3 orange 1查找出table_A的信息条件为:table_A.Quantity-table_B.Quantity>0结果为:ID Name Quantity
1 apple 5
1 apple 5
2 banana 1
3 orange 1table_BID Name Quantity
1 apple 2
2 banana 1
3 orange 1查找出table_A的信息条件为:table_A.Quantity-table_B.Quantity>0结果为:ID Name Quantity
1 apple 5
where a.Quantity-b.Quantity>0
select a.id,a.name,a.quantity from table_a a join table_b on a.id=b.id where table_A.Quantity-table_B.Quantity>0
http://www.w3school.com.cn/sql/index.asp