高效的CASE语句，SQL

Question

Here is my query:

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

<a fairly complex query> may return NULL , in which case I want to set col3 to col2 . I know I could do this with a CASE statement:

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

However, that approach executes <a fairly complex query> twice. What are some options if I only want to execute <a fairly complex query> once?

Answer 1

You could use subquery and COALESCE :

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

Without subquery:

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>