Sql 将所有列合并为一个列的查询

Question

  id college1  college2  college3
   1 abc       xyz        rst

以上是输入-

正在寻找输出：-

id college1  college2  college3
 1  abc 
 1  xyz
 1  rst

Answer 1

一种方法就是 union all:

select id, college1 as college from t
union all
select id, college2 as college from t
union all
select id, college3 as college from t;

这需要扫描table三遍，这通常没问题。但是如果 "t" 真的是一个复杂的查询或者非常大的查询 table，还有更有效的方法。

Sql query to merge all columns into a single column