SQL条件条件，如果条件不满足，则返回所有

Question

I have a similar question to SQL Conditional Where and Conditional WHERE clause in SQL Server , except when the condition is not met, I want to return all rows (as if there were no WHERE clause).

One solution could be to do the following. Note that <ColLetter> and <ColValue> are string insertions, where <ColLetter> will be one of ColA , ColB , ColC , or NULL , and <ColValue> will be a value that can appear in one of those columns (or is NULL if the <ColLetter> is).

SELECT ID, ColA, ColB, ColC
FROM MainTable t
WHERE (
  -- <ColLetter> is not null, corresponding to a column name in MainTable
  (ISNULL(<ColLetter>, 0) <> 0 AND t.<ColLetter> =  <ColValue>)
  OR
  -- <ColLetter> is null, I want to return all rows
  (ISNULL(<ColLetter>, 0) = 0 AND t.ID is not null)
  )

The issue with this is that it checks the ID column for nulls unnecessarily (there won't be any nulls) and it is a large table. Is there a better way to do this?

Answer 1

where case <colLetter>
     when '<nameOfColA>' then nameOfColA
     when '<nameOfColB>' then nameOfColB
     When '<nameOfColC>' then nameOfColC
     else isNull(<colValue>, 0) end 
      = isNull(<colValue>, 0)        

Answer 2

This boils down to

SELECT ID, ColA, ColB, ColC
FROM MainTable t
WHERE (
   -- <ColLetter> is null, I want to return all rows
   ISNULL(<ColLetter>, 0) = 0 
   -- Check the value otherwise
   OR t.<ColLetter> = <ColValue>)
  )