高效的CASE语句，SQL

Question

这是我的查询：

select col1, col2, (<a fairly complex query> as col3) from <a table>

<a fairly complex query>可能返回NULL ，在这种情况下，我想将col3设置为col2 。 我知道我可以使用CASE语句来做到这一点：

select col1, col2, 
CASE WHEN (<a fairly complex query>) is NULL col2
ELSE (<a fairly complex query>) END AS col3 
from <a table>

但是，该方法执行<a fairly complex query>两次。 如果我只想执行一次<a fairly complex query> ，有哪些选择？

Answer 1

您可以使用subquery和COALESCE ：

SELECT col1, col2, COALESCE(col3, col2) AS col3
FROM (select col1, col2, (<a fairly complex query>) as col3 
      from <a table>) AS sub;

没有子查询：

SELECT col1, col2, COALESCE(<a fairly complex query>, col2) AS col3
FROM <a table>;

Answer 2

与其他答案类似，但是不需要子查询：

select col1, col2, COALESCE(<a fairly complex query>, col2) AS col3 from <a table>