This error occurs when attempting to drop or rename a column that is referenced by a CHECK constraint. MySQL prevents the operation to maintain constraint integrity unless the constraint is dropped first.
This error was introduced in MySQL 8.0.19 as part of stricter enforcement of constraint dependencies. When you attempt to drop or rename a column using ALTER TABLE, MySQL checks if that column is used in any CHECK constraints. If a dependency is found, the operation is blocked to prevent breaking the constraint logic. CHECK constraints are validation rules that ensure data meets specific conditions before being inserted or updated. If a column used in such a constraint is removed, the constraint would become invalid and unable to enforce its rules. MySQL protects against this by requiring explicit handling of the constraint before allowing column modifications. The error message typically includes the constraint name and column name: "Check constraint 'constraint_name' uses column 'column_name', hence column cannot be dropped or renamed." This prevents accidental data integrity violations that could occur if constraints were silently broken during schema changes.
First, determine which CHECK constraints reference the column you want to drop. Use SHOW CREATE TABLE to see all constraints:
SHOW CREATE TABLE your_table_name;Look for CHECK constraint definitions in the output. You can also query the INFORMATION_SCHEMA:
SELECT CONSTRAINT_NAME, CHECK_CLAUSE
FROM INFORMATION_SCHEMA.CHECK_CONSTRAINTS
WHERE CONSTRAINT_SCHEMA = 'your_database_name'
AND TABLE_NAME = 'your_table_name';Note the names of any constraints that reference your target column.
Before dropping or renaming the column, you must drop the CHECK constraint. Use ALTER TABLE with DROP CHECK:
ALTER TABLE your_table_name
DROP CHECK constraint_name;If there are multiple constraints on the column, drop each one:
ALTER TABLE your_table_name
DROP CHECK constraint_1,
DROP CHECK constraint_2;Verify the constraints are removed by running SHOW CREATE TABLE again.
Now that the CHECK constraint is removed, you can safely modify the column:
To drop the column:
ALTER TABLE your_table_name
DROP COLUMN column_name;To rename the column:
ALTER TABLE your_table_name
RENAME COLUMN old_name TO new_name;Or using CHANGE:
ALTER TABLE your_table_name
CHANGE old_name new_name data_type;The operation should now complete without error 3959.
If you renamed the column and still need the CHECK constraint, recreate it with the new column name:
ALTER TABLE your_table_name
ADD CONSTRAINT constraint_name
CHECK (new_column_name > 0);Replace the CHECK expression with your original constraint logic, using the new column name.
For complex constraints, you may want to save the original CHECK_CLAUSE from INFORMATION_SCHEMA before dropping it, then modify and reapply it.
Single-Column Constraint Auto-Drop: MySQL has a special rule: if a CHECK constraint refers to only a single column, dropping that column automatically drops the constraint as well. However, if the constraint references multiple columns, you must manually drop the constraint first, even if you're only dropping one of the referenced columns.
Combining Operations in One Statement: You can drop the constraint and column in a single ALTER TABLE statement to make the operation atomic:
ALTER TABLE your_table_name
DROP CHECK constraint_name,
DROP COLUMN column_name;This is safer for production systems as it reduces the window where the table is in an intermediate state.
Migration Tool Considerations: ORM and migration tools (Alembic, Flyway, Liquibase, etc.) may not automatically handle CHECK constraint dependencies. When generating migrations for MySQL 8.0.16+, verify that constraint drops are included before column modifications. Some tools require explicit configuration or manual migration script editing.
Version Compatibility: CHECK constraints were introduced in MySQL 8.0.16, and the stricter dependency checking in error 3959 was added in MySQL 8.0.19. If you're migrating from MySQL 5.7 or earlier, be aware that CHECK constraints in table definitions were previously ignored, so existing schemas may need updating.
Constraint Naming: If you didn't explicitly name your CHECK constraints during creation, MySQL generates names automatically (like table_name_chk_1). Always use SHOW CREATE TABLE or INFORMATION_SCHEMA to find the actual constraint names before attempting to drop them.
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