Sql 伪IF/Case帮助

运行PostgreSQL 7.x（是的，我正在升级）

问题是：

如果没有返回数据，我需要设置三到四个字段

我在想这样的事情

SELECT CASE
       WHEN default_field IS NULL THEN field_1 = 'blah', field_2 = 'foo', field_3 = 'bar'
       ELSE field_1,field_2,field_3
       END

我该怎么做呢

所以考虑一个IF条件

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如果其他字段得到这些值，否则返回的值保持原样

我只需要独立地拼写3个case语句

SELECT CASE WHEN default_field IS NULL THEN 'blah' ELSE field_1 END AS field_1,
       CASE WHEN default_field IS NULL THEN 'foo' ELSE field_2 END AS field_2,
       CASE WHEN default_field IS NULL THEN 'bar' ELSE field_3 END AS field_3
...

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我只是独立地解释这3个案例陈述

SELECT CASE WHEN default_field IS NULL THEN 'blah' ELSE field_1 END AS field_1,
       CASE WHEN default_field IS NULL THEN 'foo' ELSE field_2 END AS field_2,
       CASE WHEN default_field IS NULL THEN 'bar' ELSE field_3 END AS field_3
...