Renaming the columns of a table columns according to a mapping defined in another table - with MYSQL

Question

I'd like to create a MYSQL stored procedure that dynamically renames the column names of TABLE_EXAMPLE with content from TABLE_MASTER -> the expected result is shown on TABLE_RESULT.

Here are the tables content:

    TABLE_EXAMPLE (THE HEADERS MIGHT CHANGE BUT THEY ARE MAPPED WITH A COLUMN IN TABLE_EXAMPLE):

           |header01 | header02|...|header n|
           |data01 ..| data02..|...|data 0n|
           |data11 ..| data12..|...|data 1n|
             ..........etc.................
           |data n1..|data n2..|...|data nn|

    TABLE_MASTER (STATIC TABLE, UNCHANGED):

           |ORIGIN| TARGET| NAME|
           |header01|header_master01|Paul|
           |header02|header_master02|Paul|
            ..........etc.................
           |header n|header_master n|Paul|

The expected result contains the data from TABLE_EXAMPLE but with mapped column names found via TABLE_MASTER.TARGET:

         TABLE_RESULT:
            |data_master01|data_master02|...|data_master0n| NAME|
            |data01.......|data02.......|...|data 0n.......|Paul|
            |data11.......|data12.......|...|data 1n.......|Paul|  
            .........................etc.........................
           |data n1..|data n2...........|...|data nn.......|Paul|

PS: A simple: "ALTER TABLE table_example CHANGE COLUMN old new char(250)", won't do.

Thanks for your help!

EDIT 1: I have tried to write this but without success because 'oldname' and 'newname' are not considered as variables.

BEGIN

DECLARE n INT(10) ; DECLARE i INT(10) ;
DECLARE oldname VARCHAR(40); DECLARE newname VARCHAR(40);

SET n=(SELECT count(id) FROM `master_table` where `name`='paul');

SET i=1; WHILE i<n DO 

SET oldname=(SELECT `COLUMN_NAME`  FROM `INFORMATION_SCHEMA`.`COLUMNS` WHERE 
`TABLE_SCHEMA`='mydb'AND `TABLE_NAME`='table_example' LIMIT 1, 1) ; 
SET newname=(SELECT TARGET FROM MASTER_TABLE WHERE ORIGIN='oldname');

ALTER TABLE `table_example` CHANGE oldname newname VARCHAR(50) NOT NULL;

SET i=i+1 ; END WHILE ;
END

Answer 1

I think I understand you want to select a column by name, and the names are strings in your TABLE_MASTER.

You can't do this in a single SQL query, because SQL cannot select a column by using a string expression. There's a difference between a string and an identifier. For example, this selects data from a column by identifier:

SELECT header01 ...

But the following is a string expression (a simple one, which is just a constant value). It returns only a fixed string 'header01', NOT the data from a column of that name:

SELECT 'header01' ...

Likewise, using any other expression in a select-list only selects the value of that expression, NOT the data stored in a column named by the string value of the expression.

Therefore if you want a query to return a dynamic column named by some other variable or expression, you can't do it in the same query where you read that expression. You have to format a new SQL query from the values you read. This is called a dynamic SQL statement (already mentioned by spencer7593, who posted an answer while I was writing my own answer).

You could use your TABLE_MASTER to format a dynamic SQL statement to fetch columns and redefine their alias:

SELECT CONCAT(
  'SELECT ', 
   GROUP_CONCAT(CONCAT(ORIGIN, ' AS ', TARGET)), ', ', 
   QUOTE(MAX(NAME)), ' AS NAME ',
  'FROM TABLE_EXAMPLE'
) INTO @sql
FROM TABLE_MASTER;

The result of this is a string that forms another SELECT statement, this one renames the columns as you want:

SELECT header01 AS header_master01,header02 AS header_master02, 'Paul' AS NAME FROM TABLE_EXAMPLE  

Then you can use the string stored in @sql as a dynamic SQL query.

Here's the procedure that does this:

DELIMITER ;;

CREATE PROCEDURE MyProc()
BEGIN
    SELECT CONCAT(
      'SELECT ', 
       GROUP_CONCAT(CONCAT(ORIGIN, ' AS ', TARGET)), ', ', 
       QUOTE(MAX(NAME)), ' AS NAME ',
      'FROM TABLE_EXAMPLE'
    ) INTO @sql
    FROM TABLE_MASTER;
    PREPARE stmt FROM @sql;
    EXECUTE stmt;
    DEALLOCATE PREPARE stmt;
END

DELIMITER ;

Call the procedure, and get the result:

CALL MyProc();

+-----------------+-----------------+------+
| header_master01 | header_master02 | NAME |
+-----------------+-----------------+------+
| data01          | data02          | Paul |
| data11          | data12          | Paul |
+-----------------+-----------------+------+

I have to comment that this is a lot of trouble to go through. I would rather fetch the data as it is in the database, and reformat it in my application code. Then I wouldn't have to use any dynamic SQL to format the columns.

Answer 2

The specification isn't entirely clear, what it is exactly we are trying to achieve.

Looks like we are wanting a stored procedure that returns a resultset.

That would mean the procedure would execute a SELECT statement.

It is possible for a SELECT statement to assign an alias to an expression returned in the SELECT list, which then becomes the column name in the resultset. For example

  SELECT 'some_expression' AS foo

The resultset returned by this SELECT statement will contain a column named foo.

The identifiers (table names, column names, function names) referenced in the SELECT statement are not dynamic; those cannot be modified dynamically in the execution of the statement. The alias to be assigned to the column must be specified in the statement

  SELECT some_expr AS new_column_name 

And have the value of new_column_name by dynamic, and retrieved from some table, we would need to run a separate SQL statement to retrieve the column name to be used.

Within the procedure, we can dynamically prepare the SQL text we want to execute.

As an example, with something like this in the body of a stored procedure :

  DECLARE col_name VARCHAR(80);

  SET col_name = 'foo' ;

  SET @sql = CONCAT('SELECT ''some_expr'' AS `',col_name,'`') ;

  PREPARE stmt FROM @sql;
  EXECUTE stmt;
  DEALLOCATE PREPARE stmt;  

Executing the procedure would cause a statement like this to be executed:

  SELECT 'some_expr' AS `foo`

returning a resultset with a column name of foo.

---

Reference: https://dev.mysql.com/doc/refman/8.0/en/sql-syntax-prepared-statements.html

Answer 3

Here is a stored procedure for the purpose of renaming the columns of table table_example according to the mapping defined in table_master. It works by fetching the content of the mapping, and then dynamically generate a series of ALTER TABLE ... RENAME COLUMN ... TO ... commands (note that this syntax is only supported in SQL 8.0).

Obviously this is a one-shot procedure, since once it completes, the column are renamed.

You uncomment the SELECT @q statement and comment the three following lines to execute the procedure and display the rename commands without actually running them.

DELIMITER $$
CREATE PROCEDURE RenameColumns()
BEGIN

    DECLARE finished INTEGER DEFAULT 0;
    DECLARE old_col VARCHAR(50);
    DECLARE new_col VARCHAR(50);

    DECLARE curs CURSOR FOR SELECT origin, target FROM table_master;
    DECLARE CONTINUE HANDLER FOR NOT FOUND SET finished = 1;

    OPEN curs;
    renameLoop: LOOP

        FETCH curs INTO old_col, new_col;
        IF finished = 1 THEN 
            LEAVE renameLoop;
        END IF;

        SET @q = CONCAT('ALTER TABLE table_example RENAME COLUMN ', old_col, ' TO ', new_col);
        -- SELECT @q;
        PREPARE stmt FROM @q;
        EXECUTE stmt;
        DEALLOCATE PREPARE stmt; 

    END LOOP renameLoop;
    CLOSE curs;

END$$
DELIMITER ;

Demo on DB Fiddle

select * from table_master;

| origin   | target          |
| -------- | --------------- |
| header01 | header_master01 |
| header02 | header_master02 |
| header03 | header_master03 |


select * from table_example;

| header01 | header02 | header03 |
| -------- | -------- | -------- |
| 1        | 2        | 3        |


CALL RenameColumns();

select * from table_example;

| header_master01 | header_master02 | header_master03 |
| --------------- | --------------- | --------------- |
| 1               | 2               | 3               |