MariaDB: Conditional Column Modifications without "Change Column If Exists"

There are two main points to consider:

1. IF EXISTS Clause: While MariaDB offers the IF EXISTS clause for other operations like dropping tables or constraints, it doesn't currently work with the CHANGE COLUMN syntax [1].

2. Alternative Approach: However, there's a workaround. You can simply execute the ALTER TABLE CHANGE COLUMN statement directly. MariaDB will attempt to modify the column. If the column doesn't exist, the statement will fail with an error. In most cases, database tools or scripts can handle these errors gracefully and retry the operation if needed.

Here's an example:

ALTER TABLE my_table CHANGE COLUMN old_column_name new_column_name new_data_type;

This tries to change the old_column_name to new_column_name with a new data type. If the column doesn't exist, the statement will raise an error.

Example 1: Handling error during execution (procedural)

-- Define the table name and desired changes
SET @table_name = 'my_table';
SET @old_column = 'old_column_name';
SET @new_column = 'new_column_name';
SET @new_type = 'VARCHAR(255)';

-- Attempt to change the column
BEGIN TRY
  ALTER TABLE @table_name CHANGE COLUMN @old_column @new_column @new_type;
  -- Success message (optional)
  SELECT 'Column modified successfully.';
END TRY
BEGIN CATCH -- Handle potential errors
  IF @@ERROR = 1054 -- Column doesn't exist error code
  THEN
    SELECT 'Column does not exist.';
  ELSE
    -- Handle other errors (optional)
    SELECT CONCAT('Error: (', @@ERROR, ') ', @@ERROR_MESSAGE);
  END IF;
END CATCH;

This code defines variables for the table name, old column name, new column name, and data type. It then uses a TRY...CATCH block to attempt the ALTER TABLE statement.

• If the column exists and the change is successful, a success message is displayed.
• If the error code indicates the column doesn't exist (error code 1054 for MariaDB), a message specifies that.
• You can optionally add logic to handle other potential errors within the CATCH block.

Example 2: Checking existence before modification (declarative)

-- Check if the column exists
SELECT COUNT(*) AS column_exists
FROM INFORMATION_SCHEMA.COLUMNS
WHERE TABLE_NAME = 'my_table'
  AND COLUMN_NAME = 'old_column_name';

-- Modify the column only if it exists (procedural logic can be implemented here)
IF column_exists > 0 THEN
  ALTER TABLE my_table CHANGE COLUMN old_column_name new_column_name VARCHAR(255);
END IF;

This example first uses a query to check if the old_column_name exists in the my_table using information schema tables.

• If the column_exists count is greater than 0, the column exists, and the ALTER TABLE statement proceeds to modify it.
• This approach allows for more flexibility in handling the existence check. You can integrate this logic into a larger script or procedure.

1. Stored Procedures: Create a stored procedure that encapsulates the logic for checking column existence and modification. This allows for modularity and reusability across your code.

2. User-Defined Functions (UDFs): Similar to stored procedures, you can write a UDF that takes the table name and column name as arguments. It can then check existence and return a success flag or perform the modification logic.

3. Schema Versioning Tools: If you're using schema versioning tools for managing database changes, these tools might have built-in functionalities for handling "if not exists" scenarios during schema migrations. Explore the capabilities of your chosen tool.

4. Pre-check with SHOW COLUMNS: Before attempting the ALTER TABLE statement, use a SHOW COLUMNS FROM my_table query to retrieve the existing columns. If the desired column isn't present in the results, you can handle the non-existence gracefully.