How to store the result of dynamic SQL in a variable?

Question

I have the following theoretical statement which I would like to implement using dynamic SQL (on SQL Server 2016) and store the single output value in the variable (@output).

@numericvar, @columnname, @tablename should be input parameters. Any help would be greatly appreciated. Many thanks.

SELECT @output = (
    SELECT 
        MAX(CASE WHEN ROWNUM*1.0/NUMROWS <= @numericvar THEN @columnname END)
    FROM (
        SELECT
            @columnname,
            ROW_NUMBER() OVER (ORDER BY @columnname ) AS ROWNUM,
            COUNT(*) OVER (PARTITION BY NULL) AS NUMROWS
        FROM 
            @tablename 
    ) @tablename 
); 

Answer 1

DECLARE @columnname SYSNAME, @tablename SYSNAME, @numericvar NUMERIC(18,2);
DECLARE @output NUMERIC(18,2);

DECLARE @sql NVARCHAR(MAX) = N'
    SET @output = (
        SELECT 
            MAX(CASE WHEN ROWNUM*1.0/NUMROWS <= @numericvar THEN '+QUOTENAME(@columnname)+N' END)
        FROM (
            SELECT
                '+QUOTENAME(@columnname)+N',
                ROW_NUMBER() OVER (ORDER BY '+QUOTENAME(@columnname)+N' ) AS ROWNUM,
                COUNT(*) OVER (PARTITION BY NULL) AS NUMROWS
            FROM 
                '+QUOTENAME(@tablename)+N'
        ) AS t
    );
';

EXECUTE sp_executesql 
    @sql,
    N'@numericvar NUMERIC(18,2), @output NUMERIC(18,2) OUTPUT', 
    @numericvar, @output OUTPUT;

SELECT @output;

---

Update: a working example for FLOAT output. The script uses a table in the INFORMATION_SCHEMA schema which everyone has.

See if you can make it work from this sample. If you can't I suggest you edit your question, and add the exact script + parameter values you are using + indication of the type of columnname.

DECLARE @schemaname SYSNAME='INFORMATION_SCHEMA',
        @tablename  SYSNAME='COLUMNS', 
        @columnname SYSNAME='NUMERIC_PRECISION', 
        @numericvar NUMERIC(18,2)=.5;

DECLARE @output_f FLOAT;

DECLARE @sql NVARCHAR(MAX) = N'
    SET @output_f = (
            SELECT 
                MAX(CASE WHEN ROWNUM*1.0/NUMROWS<=@numericvar THEN '+QUOTENAME(@columnname)+N' END)
            FROM (
                SELECT
                    '+QUOTENAME(@columnname)+N',
                    ROW_NUMBER() OVER (ORDER BY '+QUOTENAME(@columnname)+N') AS ROWNUM,
                    COUNT(*) OVER () AS NUMROWS
                FROM 
                    '+QUOTENAME(@schemaname)+N'.'+QUOTENAME(@tablename)+N'
            ) AS t
    );
';

EXECUTE sp_executesql 
    @sql,
    N'@numericvar NUMERIC(18,2), @output_f FLOAT OUTPUT', 
    @numericvar, @output_f OUTPUT;

SELECT @output_f;