mysql - 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>可能会返回 NULL ，在这种情况下我想设置 col3至 col2 .我知道我可以用 CASE 做到这一点声明:

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 <a fairly complex query> 有哪些选项？一次？

Answer 1

您可以使用子查询和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>;