Found a couple of similar questions here on this, but couldn't figure out how to apply to my scenario.
My function has a parameter called @IncludeBelow. Values are 0 or 1 (BIT).
I have this query:
SELECT p.*
FROM Locations l
INNER JOIN Posts p
on l.LocationId = p.LocationId
WHERE l.Condition1 = @Value1
AND l.SomeOtherCondition = @SomeOtherValue
If @IncludeBelow is 0, i need the query to be this:
SELECT p.*
FROM Locations l
INNER JOIN Posts p
on l.LocationId = p.LocationId
WHERE l.Condition1 = @Value1
AND l.SomeOtherCondition = @SomeOtherValue
AND p.LocationType = @LocationType -- additional filter to only include level.
If @IncludeBelow is 1, that last line needs to be excluded. (i.e don't apply filter).
I'm guessing it needs to be a CASE statement, but can't figure out the syntax.
Here's what i've tried:
SELECT p.*
FROM Locations l
INNER JOIN Posts p
on l.LocationId = p.LocationId
WHERE l.Condition1 = @Value1
AND l.SomeOtherCondition = @SomeOtherValue
AND (CASE @IncludeBelow WHEN 0 THEN p.LocationTypeId = @LocationType ELSE 1 = 1)
Obviously that's not correct.
What's the correct syntax?
I changed the query to use EXISTS because if there's more than one location associated with a POST, there'd be duplicate POST records that'd require a DISTINCT or GROUP BY clause to get rid of...
The non-sargable
This will perform the worst of the possible solutions:
SELECT p.*
FROM POSTS p
WHERE EXISTS(SELECT NULL
FROM LOCATIONS l
WHERE l.LocationId = p.LocationId
AND l.Condition1 = @Value1
AND l.SomeOtherCondition = @SomeOtherValue)
AND (@IncludeBelow = 1 OR p.LocationTypeId = @LocationType)
The sargable, non-dynamic version
Self explanitory....
BEGIN
IF @IncludeBelow = 0 THEN
SELECT p.*
FROM POSTS p
WHERE EXISTS(SELECT NULL
FROM LOCATIONS l
WHERE l.LocationId = p.LocationId
AND l.Condition1 = @Value1
AND l.SomeOtherCondition = @SomeOtherValue)
AND p.LocationTypeId = @LocationType
ELSE
SELECT p.*
FROM POSTS p
WHERE EXISTS(SELECT NULL
FROM LOCATIONS l
WHERE l.LocationId = p.LocationId
AND l.Condition1 = @Value1
AND l.SomeOtherCondition = @SomeOtherValue)
END
The sargable, dynamic version (SQL Server 2005+):
Love or hate it, dynamic SQL lets you write the query once. Just be aware that sp_executesql caches the query plan, unlike EXEC in SQL Server. Highly recommend reading The Curse and Blessings of Dynamic SQL before considering dynamic SQL on SQL Server...
DECLARE @SQL VARCHAR(MAX)
SET @SQL = 'SELECT p.*
FROM POSTS p
WHERE EXISTS(SELECT NULL
FROM LOCATIONS l
WHERE l.LocationId = p.LocationId
AND l.Condition1 = @Value1
AND l.SomeOtherCondition = @SomeOtherValue)'
SET @SQL = @SQL + CASE
WHEN @IncludeBelow = 0 THEN
' AND p.LocationTypeId = @LocationType '
ELSE ''
END
BEGIN
EXEC sp_executesql @SQL,
N'@Value1 INT, @SomeOtherValue VARCHAR(40), @LocationType INT',
@Value1, @SomeOtherValue, @LocationType
END
You can write it as
SELECT p.*
FROM Locations l
INNER JOIN Posts p
ON l.LocationId = p.LocationId
WHERE l.Condition1 = @Value1
AND l.SomeOtherCondition = @SomeOtherValue
AND ((@IncludeBelow = 1) OR (p.LocationTypeId = @LocationType))
which is a pattern you see a lot e.g. for optional search parameters. But IIRC that can mess up the query execution plans so there may be a better way to do this.
Since it's only a bit, it almost might be worth deciding between two blocks of SQL with or without the check, e.g. using an IF in a stored procedure or with different command strings in calling code, based on the bit?
You can change your CASE statement to this. The query planner sees this differently, but it may be no more efficient than using OR:
(p.LocationTypeId = CASE @IncludeBelow WHEN 0 THEN p.LocationTypeId ELSE @LocationType END)
来源：https://stackoverflow.com/questions/4485965/t-sql-conditional-where-clause