SQL Server Stored Procedure - Executing different queries by CASE - sql-server

Question

I have two parameters for my stored procedure. Based on what the value of Searching_Condition is, the proper column must be searched. In a pseudo code format, it should be something like this
//CASE #Search_Condition
// WHEN 'UserID' THEN SELECT * FROM user_table WHERE UserID LIKE '#Keywords'
// WHEN 'UserName' THEN SELECT * FROM user_table WHERE UserName LIKE '#Keywords'
// WHEN 'UserAddress' THEN SELECT * FROM user_table WHERE UserAddress LIKE '#Keywords'
The following is the code I was working on and where got stuck. It should be simple but man... for being not familiar with SQL Server, I'm so struggling with it and CASE in SQL Server doesn't work the way I thought it would.
Thanks !
CREATE PROCEDURE [dbo].[USP_SP_NAME]
#Searching_Condition NVARCHAR(100),
#Keywords NVARCHAR(100)
AS
SET NOCOUNT ON
SET LOCK_TIMEOUT 3000
SET XACT_ABORT ON
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED
BEGIN TRY
SELECT
CASE WHEN
#Searching_Condition = 'user_id' THEN
(select count(*) from user_table)
WHEN
#Searching_Condition = 'user_name' THEN
(select * from user_table)
END
END TRY

Answer 1

The key concept that will help you get this right is the difference between expressions and statements.
A statement is procedural and directs the flow of control. You can think of an instruction pointer proceeding from statement to statement, and every statement is isolated from other statements (although they can select which statements after them are executed or not). They can be thought of as verbs.
An expression is something that reduces to a value--a scalar value, a string, or even a rowset--but the expression doesn't command to DO anything. They can be thought of as nouns. These nouns can't exist by themselves, they must be in the context of a statement.
The CASE statement in SQL Server is an expression. It isn't a procedural statement like Select Case is in, for example, Visual Basic. And the trick is, when the language expects an expression, you cannot substitute a statement--and furthermore, except in some special usages, you can't put procedural statements in the middle of expressions (except rowsets that can be evaluated as an expression, such as a single-column and single-row SELECT, or an EXISTS). An expression can contain expressions which contain expressions. They're like a tree, that is collapsed in order all the way down.
Think of the parts in EXECUTE dbo.MyStoredProcedure (8 + ##SPID) / 2: this is a single statement, with one parameter expression, consisting of three sub-expressions, evaluated in a certain order, that resolve to a single value, which is used as an argument to the stored procedure. You could not execute (8 + ##SPID) / 2 by itself, because it isn't a statement. (Never mind that the expression is silly, it is just for example.)
I did say that in some cases rowsets can be values, but the expected type of almost all expressions is a single value--not a rowset. That's the problem that's happening here--your outer SELECT statement is expecting a single value for the definition of the first column in a single row (since you have no FROM clause), but you're trying to provide a whole rowset when your searching condition is 'user_name'.
You can solve this by abandoning CASE entirely and using IF--because IF is a procedural statement.
CREATE PROCEDURE [dbo].[USP_SP_NAME]
#Searching_Condition NVARCHAR(100),
#Keywords NVARCHAR(100)
AS
SET NOCOUNT ON;
SET LOCK_TIMEOUT 3000;
SET XACT_ABORT ON;
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED;
BEGIN TRY
IF #Searching_Condition = 'user_id' BEGIN
select count(*) from user_table;
END
ELSE IF #Searching_Condition = 'user_name' BEGIN
select * from user_table;
END;
END TRY;
I advocate avoiding the version of the IF that doesn't use BEGIN and END and accepts a single statement--this form leads to confusion and bugs. I use BEGIN and END every time, which seems like a pain, until you discover how much time and effort doing so saves you down the road...

Answer 2

You can try this, no need to give cases, where condition will change depending on the value of #Searching_Condition:
CREATE PROCEDURE [dbo].[USP_SP_NAME]
#Searching_Condition NVARCHAR(100),
#Keywords NVARCHAR(100)
AS
SET NOCOUNT ON
SET LOCK_TIMEOUT 3000
SET XACT_ABORT ON
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED
BEGIN TRY
exec('Select * from user_table WHERE'+ #Searching_Condition+' LIKE '+ #Keywords);
END TRY