向 where 子句添加条件

Question

How would I write a sql statement that only includes conditions in a where clause if the param has a value (i only want to include the field2 check in the where clause if the param has a valid value)

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  

Answer 1

A typical method uses or with explicit NULL comparisons for the parameter:

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) ;

If you specifically wanted white space to behave the same as NULL s, then this gets more complicated. Something like:

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) ;

However, I think it would be more common to turn blank strings into NULL s before passing in the parameter, so the SQL would only be working with NULL as a special value.