Select statement defined by parameter SQL - mysql

Question

In SQL is there a way to grab the information in a table, but with the table name being specified by a function parameter?
Obviously the following doesn't work, but something along these lines maybe:
CREATE OR REPLACE FUNCTION select_table(table_name TEXT)
RETURNS TABLE (
"ID" TEXT
) AS
$$
SELECT * FROM uploads.<table_name>
$$
LANGUAGE SQL STABLE;
I'm a bit of a rookie when it comes to SQL, so would appreciate any guidance.

Answer 1

Any use of a variable in a query will be as if you had used a string literal, not an identifier.
To use a variable as an identifier, you would have to use dynamic SQL. That is, do string-concatenation of the table_name variable into a string which is your SELECT query, then PREPARE and EXECUTE that query.
But https://dev.mysql.com/doc/refman/8.0/en/sql-prepared-statements.html says:
SQL syntax for prepared statements can be used within stored procedures, but not in stored functions or triggers.
This is because if a stored function is running, then by definition, your thread is already running a query. MySQL cannot do that.
You can write your function to do a large CASE statement to do a different fixed query depending on the input variable.
But as P.Salmon comments above, in MySQL you can't return a table from a stored function. Functions can only return a single scalar value, not a result set. So your SELECT would need to query one column and use INTO syntax to save it to a variable. Then return that variable.
Also current versions of MySQL have no "CREATE OR REPLACE FUNCTION" option. You can "CREATE FUNCTION."
DELIMITER //
CREATE FUNCTION select_table(table_name TEXT)
RETURNS TEXT READS SQL DATA
BEGIN
DECLARE result TEXT;
CASE table_name
WHEN 'mytable1' THEN SELECT col1 INTO result FROM mytable1;
WHEN 'mytable2' THEN SELECT col1 INTO result FROM mytable2;
WHEN 'mytable3' THEN SELECT col1 INTO result FROM mytable3;
END CASE;
RETURN result;
END//
DELIMITER ;