1/7/2024 0 Comments Mysql on delete cascade![]() ![]() However, the ON UPDATE CASCADE, ON UPDATE SET NULL, ON DELETE SET NULL clauses are not allowed in this case. For integer types, the size and sign must also be the same.īoth the foreign key columns and the referenced columns can be PERSISTENT columns. The foreign key columns and the referenced columns must be of the same type, or similar types. The referenced columns in the parent table must be a an index or a prefix of an index. If MariaDB automatically creates an index for the foreign key (because it does not exist and is not explicitly created), its name will be index_name. Index prefixes are not supported (thus, TEXT and BLOB columns cannot be used as foreign keys). The columns in the child table must be a BTREE (not HASH, RTREE, or FULLTEXT - see SHOW INDEX) index, or the leftmost part of a BTREE index. The symbol clause, if specified, is used in error messages and must be unique in the database. RESTRICT | CASCADE | SET NULL | NO ACTION | SET DEFAULT The definition must follow this syntax: ] FOREIGN KEY See the Examples below.įoreign keys are created with CREATE TABLE or ALTER TABLE. However, only the syntax described below creates foreign keys.įrom MariaDB 10.5, MariaDB will attempt to apply the constraint. MariaDB simply parses it without returning any error or warning, for compatibility with other DBMS's. For example: CREATE TABLE b(for_key INT REFERENCES a(not_key)) Each option has distinct implications for data integrity, query complexity, and database size.Note: Until MariaDB 10.4, MariaDB accepts the shortcut format with a REFERENCES clause only in ALTER TABLE and CREATE TABLE statements, but that syntax does nothing. In conclusion, the choice between ‘ON DELETE CASCADE’ and ‘ON DELETE SET NULL’ in database design depends on the specific requirements of the application and the desired behavior when dealing with parent and child records. Retains child records, potentially leading to a smaller reduction in database size. May lead to a more significant reduction in database size due to cascading deletions. Requires additional consideration when querying to handle NULL foreign key values. Simplifies query, as there is no need to separately handle child deletions. ) Difference Between ON DELETE CASCADE and ON DELETE SET NULL in DBMSįoreign key values in child records are set to NULLĮnsures referential integrity by removing dependent records.Įnsures referential integrity by maintaining child records with NULL references. Example SQL Syntax CREATE TBALE parent_table_p (įOREIGN KEY (parent_id) REFERENCES parent_table_p(id) ON DELETE SET NULL This is another way to maintain reference integrity, allowing the child records to exist but with the NULL reference if the parent record is deleted. If you made the child table (referencing table) “ON DELETE SET NULL” this means if a record in the parent table (referenced table) is deleted then the corresponding entry or values in the child table (referencing table) will be set to NULL. The phrase “ON DELETE SET NULL” is not a standard option in most relational databases.
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