向 where 子句添加条件

Posted 2023-03-22

【中文标题】向 where 子句添加条件

【英文标题】：Adding conditions to where clause

【发布时间】：2021-07-19 11:58:47

【问题描述】：

如果参数具有值，我将如何编写仅在 where 子句中包含条件的 sql 语句（如果参数具有有效值，我只想在 where 子句中包含 field2 检查）

SELECT *  FROM abc
WHERE if(:param1 has a value that is not null or white space) then field1 = :param1   
AND   if(:param2 has a value that is not null or white space) then field2 = :param2   
AND   if(:param3 has a value that is not null or white space) then field3 = :param3   
AND   if(:param4 has a value that is not null or white space) then field4 = :param4  
AND   if(:param5 has a value that is not null or white space) then field5 = :param5  

【问题讨论】：

(field1 = :param1 or :param1 is null or :param1 = ' ')

不太理解“或”的逻辑。你能简单解释一下吗？

【参考方案1】：

一种典型的方法是使用or 和显式NULL 比较参数：

SELECT *
FROM abc
WHERE (:param1 is null OR field1 = :param1) AND
      (:param2 is null OR field2 = :param2) AND
      (:param3 is null OR field3 = :param3) AND
      (:param4 is null OR field4 = :param4) AND
      (:param5 is null OR field5 = :param5) ;

如果您特别希望空白的行为与NULLs 相同，那么这会变得更加复杂。比如：

WHERE (NULLIF(REPLACE(:param1, ' '), '') is null OR field1 = :param1) AND
      (NULLIF(REPLACE(:param2, ' '), '') is null OR field2 = :param2) AND
      (NULLIF(REPLACE(:param3, ' '), '') is null OR field3 = :param3) AND
      (NULLIF(REPLACE(:param4, ' '), '') is null OR field4 = :param4) AND
      (NULLIF(REPLACE(:param5, ' '), '') is null OR field5 = :param5) ;

但是，我认为在传入参数之前将空白字符串转换为NULLs 会更常见，因此 SQL 只能使用 NULL 作为特殊值。