Dynamic sql to populate a stored procedure variable - sql

Question

I want to add a record in a aTable using a stored procedure. But before that I need to check if it's a duplicate entry. If its not duplicate then only add to aTable.
I have one stored procedure that does two things
checking for duplicate
and if not duplicate then only add the record.
In this stored procedure, I have
#count variable (scope is the stored procedure itself)
column values as parameters (7-8 as varchar datatype)
I am setting #count to 1 and then building the dynamic #qry string that will hold the #count value as follows using #column values
CREATE PROCEDURE sp_aTable_ADD
#col1 varchar(20) = null,
#col2 varchar(20) = null,
#col3 varchar(20) = null,
...
#colN varchar(20) = null,
AS
DECLARE #count int
SET #count = 1
DECLARE #qry nvarchar(max)
SET #qry = 'SELECT #count = count(*) FROM aTable WHERE '
IF #col1 IS NOT EMPTY
BEGIN
SET #qry = #qry + ' col1 = '+ ''''+#col1+''''
END
IF #col2 IS NOT EMPTY
BEGIN
SET #qry = #qry + ' AND col2 = '+ ''''+#col2+''''
END
...
...
IF #colN IS NOT EMPTY
BEGIN
SET #qry = #qry + ' colN = '+ ''''+#colN+''''
END
/*once all the stored procedure variables are done processing, close the sql*/
SET #qry = #qry + ' )'
/*final #qry value would end up looking like as follows
SELECT #count = count(*)
FROM aTable
WHERE (col1 = 'val1' AND col2 = 'val2' AND col3 = 'val3')*/
Now, within this stored procedure I want to know the value of #count and if it is '0' then continue with the 'INSERT' procedure. I am stuck at getting '#count' value within this stored procedure.
Any thoughts?

Answer 1

Why do you want to use dynamic sql for this just Write a query with a WHERE Clause something like this
CREATE PROCEDURE usp_aTable_ADD --<-- Use usp
#col1 varchar(20) = null,
#col2 varchar(20) = null,
#col3 varchar(20) = null,
...
#colN varchar(20) = null,
AS
BEGIN
SET NOCOUNT ON;
DECLARE #Count INT;
SELECT #count = count(*)
FROM aTable
WHERE
(#col1 IS NULL OR Col1 = #col1)
AND (#col2 IS NULL OR Col2 = #col2)
AND (#col3 IS NULL OR Col3 = #col3)
.
.
AND (#colN IS NULL OR ColN = #colN)
--And then at the end of your Proc just return the #Count Variable
RETURN #count;
END

Answer 2

Use an output parameter:
EXECUTE sp_executesql #qry, N'#Count INT OUTPUT', #Count OUTPUT;
For a simple test:
DECLARE #count INT;
DECLARE #qry NVARCHAR(MAX) = 'SELECT #Count = 10';
EXECUTE sp_executesql #qry, N'#Count INT OUTPUT', #Count OUTPUT;
SELECT #Count;
Finally, you never open the parentheses in your WHERE Clause:
SET #qry = 'SELECT #count = count(*) FROM aTable WHERE '
But you go on to close them:
SET #qry = #qry + ' )'
Also, if all your parameters are null even if you did open the parentheses you will get an incorrect syntax error since your SQL will be:
SELECT #count = count(*) FROM aTable WHERE ();
Or if #Col1 is NULL You will get:
SELECT #count = count(*) FROM aTable WHERE AND col2 = col2();
You can avoid this by firstly removing the parenthesis. WHERE A = 1 means exactly the same as WHERE (A = 1), and by adding a meaningless where first:
SET #qry = 'SELECT #count = count(*) FROM aTable 1 = 1 ';
IF #col1 IS NOT EMPTY
BEGIN
SET #qry = #qry + ' AND col1 = '+ ''''+#col1+''''
END
Finally I would recommend further parameterising your query.
SET #qry = 'SELECT #count = count(*) FROM aTable WHERE 1 = 1 '
DECLARE #ParamDef NVARCHAR(MAX) = '#Col1 VARCHAR(20), #Col2 VARCHAR(20), #Col3 VARCHAR(20), #Count INT OUTPUT';
IF #Col1 IS NOT NULL
SET #qry = #qry + ' AND Col1 = #Col1';
IF #Col2 IS NOT NULL
SET #qry = #qry + ' AND Col2 = #Col2';
IF #Col3 IS NOT NULL
SET #qry = #qry + ' AND Col3 = #Col3';
EXECUTE sp_executesql #qry, #ParamDef, #Col1, #Col2, #Col3, #Count OUTPUT;
Working Examples on SQL Fiddle