Fix INSERT ... SELECT ... edge case in non-strict mode

[JanJakes]

In MySQL non-STRICT mode, when inserting from a SELECT query:

When a column is declared as NOT NULL, inserting a NULL value saves an IMPLICIT DEFAULT value instead. This behavior only applies to the INSERT ... SELECT syntax (not VALUES or SET).

In other words:

CREATE TABLE t (value INT NOT NULL);

-- Strict mode:
INSERT INTO t VALUES (NULL); -- error
INSERT INTO t SET value = NULL; -- error
INSERT INTO t SELECT NULL; -- error
INSERT INTO t VALUES ((SELECT NULL)); -- error

-- Non-strict mode:
INSERT INTO t VALUES (NULL); -- error
INSERT INTO t SET value = NULL; -- error
INSERT INTO t SELECT NULL; -- OK, saves 0 (the implicit default for integer)
INSERT INTO t VALUES ((SELECT NULL)); -- error

This fixes some failures in the activation of the top 100 plugins.

[adamziel]

How are you even coming up with these nuances. 🤯

[adamziel]

Is that SQLite coalesce? Will it give us the result we need?

[JanJakes]

@adamziel Yes. There is a very similar logic for UPDATE SET ... = NULL already:

sqlite-database-integration/wp-includes/sqlite-ast/class-wp-sqlite-driver.php

Lines 4727 to 4738 in d982450

	/*
	* In MySQL non-STRICT mode, when a column is declared as NOT NULL,
	* updating to a NULL value saves an IMPLICIT DEFAULT value instead.
	* This behavior does not apply to ON DUPLICATE KEY UPDATE clauses.
	*/
	$is_on_duplicate_key_update = 'insertUpdateList' === $parent_node->rule_name;
	if ( ! $is_strict_mode && ! $is_nullable && ! $is_on_duplicate_key_update ) {
	$implicit_default = self::DATA_TYPE_IMPLICIT_DEFAULT_MAP[ $data_type ] ?? null;
	if ( null !== $implicit_default ) {
	$value = sprintf( 'COALESCE(%s, %s)', $value, $this->connection->quote( $implicit_default ) );
	}
	}

[JanJakes]

How are you even coming up with these nuances. 🤯

@adamziel Going through the few remaining errors from the top 100 plugins 🙂