use table name as parameter in sql function - sql-server

Question

I want create function, which use table name as parameter. As I search I need use dynamic sql. I try such code:
CREATE FUNCTION get_column_id
(
#TableName VARCHAR(30),
#ColumnName VARCHAR(30),
)
RETURNS int
AS
BEGIN
IF EXISTS
(
DECLARE #SQL VARCHAR(50)
SET #sql = 'SELECT' + #ColumnName + 'FROM' + #TableName + 'WHERE #ColumnName = #ColumnNameValue';
EXEC(#sql)
)
BEGIN
But get errors. Is where any way to procceed this?
I try use dynamic sql in such way
DECLARE #SQL VARCHAR(50)
SET #SQL = 'SELECT' + #ColumnName + 'FROM' + #Table + 'WHERE #ColumnName = #ColumnNameValue'
EXEC(#SQL)
DECLARE #TableName table (Name VARCHAR(30))
INSERT INTO #TableName VALUES (#SQL)
IF EXISTS
(SELECT Name FROM #TableName WHERE Name = #ColumnNameValue)
But get Invalid use of a side-effecting operator 'EXECUTE STRING' within a function.
Does anyone knows how bypass this constraint?

Answer 1

The error is the concatenation of string which lacks space in between,
SET #sql = 'SELECT ' + #ColumnName + ' FROM ' + #TableName + ' WHERE ' + #ColumnName + ' = ' + #ColumnNameValue;
-- ^ SPACE HERE ^ ^ ^ and here
if for instance the data type of the column is string, you need to wrap the value with single quotes,
SET #sql = 'SELECT ' + #ColumnName + ' FROM ' + #TableName + ' WHERE ' + #ColumnName + ' = ''' + #ColumnNameValue + '''';
UPDATE 1
You also need to declare the parameter #ColumnNameValue, eg
CREATE FUNCTION get_column_id
(
#TableName VARCHAR(30),
#ColumnName VARCHAR(30),
#ColumnNameValue VARCHAR(30)
)

Answer 2

A UDF (user defined function) in Sql Server must be deterministic. Beside your syntax errors you won't be able to accomplish your task.
if you check this article on MSDN:
http://msdn.microsoft.com/en-us/library/ms178091.aspx
You can see the citation below:
Deterministic functions always return the same result any time they are called
with a specific set of input values and given the same state of the database.
Nondeterministic functions may return different results each time they are
called with a specific set of input values even if the database state that
they access remains the same.