mysql中insert与select的嵌套使用_MySQL
如何在mysql从多个表中组合字段然后插入到一个新表中,通过一条sql语句实现。具体情形是:有三张表a、b、c,现在需要从表b和表c中分别查几个字段的值插入到表a中对应的字段。对于这种情况,我们可以使用如下的语句来实现:
<strong>INSERT INTO</strong> db1_name(field1,field2) <strong>SELECT</strong> field1,field2 <strong>FROM</strong> db2_name
当然,上面的语句比较适合两个表的数据互插,如果多个表就不适应了。对于多个表,我们可以先将需要查询的字段join起来,然后组成一个视图后再select from就可以了:
<strong>INSERT INTO</strong> a(field1,field2) <strong>SELECT</strong> * <strong>FROM</strong>(<strong>SELECT</strong> f1,f2 <strong>FROM</strong> b <strong>JOIN</strong> c) <strong>AS</strong> tb
其中f1是表b的字段,f2是表c的字段,通过join查询就将分别来自表b和表c的字段进行了组合,然后再通过select嵌套查询插入到表a中,这样就满足了我们这个场景了,如果需要不止2个表,那么可以多个join的形式来组合字段。需要注意的是嵌套查询部分最后一定要有设置表别名,如下:
1<strong>SELECT</strong> * <strong>FROM</strong>(<strong>SELECT</strong> f1,f2 <strong>FROM</strong> b <strong>JOIN</strong> c) <strong>AS</strong> tb
即最后的as tb是必须的(当然tb这个名称可以随意取),即指定一个别名,否则在mysql中会报如下错误:
ERROR 1248 (42000): Every derived <strong>TABLE</strong> must have its own alias
即每个派生出来的新表都必须指定别名才可以的。
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