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IF… ELSE clause is very handy and whenever you need to perform any conditional operation, you can achieve your results using it. But there are some limitations in IF… ELSE, and one of the limitations is that you cannot use it in WHERE clause.
You can specify multiple conditions in a single WHERE clause to, say, retrieve rows based on the values in multiple columns. You can use the AND and OR operators to combine two or more conditions into a compound condition. AND, OR, and a third operator, NOT, are logical operators.
A query can contain both a WHERE clause and a HAVING clause. In that case: The WHERE clause is applied first to the individual rows in the tables or table-valued objects in the Diagram pane. Only the rows that meet the conditions in the WHERE clause are grouped.
Yes, PL/SQL allows us to nest if statements within if-then statements. i.e, we can place an if then statement inside another if then statement. if (condition1) then -- Executes when condition1 is true if (condition2) then -- Executes when condition2 is true end if; end if; SQL.
Use a CASE statement
UPDATE: The previous syntax (as pointed out by a few people) doesn't work. You can use CASE as follows:
WHERE OrderNumber LIKE
CASE WHEN IsNumeric(@OrderNumber) = 1 THEN
@OrderNumber
ELSE
'%' + @OrderNumber
END
Or you can use an IF statement like @N. J. Reed points out.
You should be able to do this without any IF or CASE
WHERE
(IsNumeric(@OrderNumber) AND
(CAST OrderNumber AS VARCHAR) = (CAST @OrderNumber AS VARCHAR)
OR
(NOT IsNumeric(@OrderNumber) AND
OrderNumber LIKE ('%' + @OrderNumber))
Depending on the flavour of SQL you may need to tweak the casts on the order number to an INT or VARCHAR depending on whether implicit casts are supported.
This is a very common technique in a WHERE clause. If you want to apply some "IF" logic in the WHERE clause all you need to do is add the extra condition with an boolean AND to the section where it needs to be applied.
You don't need a IF statement at all.
WHERE
(IsNumeric(@OrderNumber) = 1 AND OrderNumber = @OrderNumber)
OR (IsNumeric(@OrderNumber) = 0 AND OrderNumber LIKE '%' + @OrderNumber + '%')
There isn't a good way to do this in SQL. Some approaches I have seen:
1) Use CASE combined with boolean operators:
WHERE
OrderNumber = CASE
WHEN (IsNumeric(@OrderNumber) = 1)
THEN CONVERT(INT, @OrderNumber)
ELSE -9999 -- Some numeric value that just cannot exist in the column
END
OR
FirstName LIKE CASE
WHEN (IsNumeric(@OrderNumber) = 0)
THEN '%' + @OrderNumber
ELSE ''
END
2) Use IF's outside the SELECT
IF (IsNumeric(@OrderNumber)) = 1
BEGIN
SELECT * FROM Table
WHERE @OrderNumber = OrderNumber
END ELSE BEGIN
SELECT * FROM Table
WHERE OrderNumber LIKE '%' + @OrderNumber
END
3) Using a long string, compose your SQL statement conditionally, and then use EXEC
The 3rd approach is hideous, but it's almost the only think that works if you have a number of variable conditions like that.
Use a CASE statement instead of IF.
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