Index: src/wp-includes/wp-db.php
===================================================================
--- src/wp-includes/wp-db.php	(revision 30587)
+++ src/wp-includes/wp-db.php	(working copy)
@@ -566,10 +566,18 @@
 	 * @access protected
 	 * @var array
 	 */
-	protected $incompatible_modes = array( 'NO_ZERO_DATE', 'ONLY_FULL_GROUP_BY',
-		'STRICT_TRANS_TABLES', 'STRICT_ALL_TABLES', 'TRADITIONAL' );
+	protected $incompatible_modes = array( 'NO_ZERO_DATE', 'ONLY_FULL_GROUP_BY', 'TRADITIONAL' );
 
 	/**
+	 * A list of required SQL modes.
+	 *
+	 * @since 4.1.0
+	 * @access protected
+	 * @var array
+	 */
+	protected $required_modes = array( 'STRICT_ALL_TABLES' );
+
+	/**
 	 * Whether to use mysqli over mysql.
 	 *
 	 * @since 3.9.0
@@ -778,31 +786,12 @@
 	 */
 	public function set_sql_mode( $modes = array() ) {
 		if ( empty( $modes ) ) {
-			if ( $this->use_mysqli ) {
-				$res = mysqli_query( $this->dbh, 'SELECT @@SESSION.sql_mode' );
+			$modes = $this->get_var( "SELECT @@SESSION.sql_mode" );
+			if ( $modes ) {
+				$modes = $original_modes = explode( ',', $modes );
 			} else {
-				$res = mysql_query( 'SELECT @@SESSION.sql_mode', $this->dbh );
+				$modes = $original_modes = array();
 			}
-
-			if ( empty( $res ) ) {
-				return;
-			}
-
-			if ( $this->use_mysqli ) {
-				$modes_array = mysqli_fetch_array( $res );
-				if ( empty( $modes_array[0] ) ) {
-					return;
-				}
-				$modes_str = $modes_array[0];
-			} else {
-				$modes_str = mysql_result( $res, 0 );
-			}
-
-			if ( empty( $modes_str ) ) {
-				return;
-			}
-
-			$modes = explode( ',', $modes_str );
 		}
 
 		$modes = array_change_key_case( $modes, CASE_UPPER );
@@ -816,18 +805,32 @@
 		 */
 		$incompatible_modes = (array) apply_filters( 'incompatible_sql_modes', $this->incompatible_modes );
 
-		foreach( $modes as $i => $mode ) {
-			if ( in_array( $mode, $incompatible_modes ) ) {
-				unset( $modes[ $i ] );
+		/**
+		 * Filter the list of required SQL modes to include.
+		 *
+		 * @since 4.1.0
+		 *
+		 * @param array $required_modes An array of required modes.
+		 */
+		$required_modes = (array) apply_filters( 'required_sql_modes', $this->required_modes );
+
+		$modes = array_diff( $modes, $incompatible_modes );
+		$modes = array_unique( array_merge( $modes, $required_modes ) );
+
+		// Don't run SET SESSION if we have nothing to change.
+		if ( isset( $original_modes ) ) {
+			sort( $original_modes );
+			sort( $modes );
+			if ( $original_modes === $modes ) {
+				return;
 			}
 		}
 
 		$modes_str = implode( ',', $modes );
 
-		if ( $this->use_mysqli ) {
-			mysqli_query( $this->dbh, "SET SESSION sql_mode='$modes_str'" );
-		} else {
-			mysql_query( "SET SESSION sql_mode='$modes_str'", $this->dbh );
+		$this->query( "SET SESSION sql_mode='$modes_str'" );
+		if ( $this->last_error ) {
+			dead_db();
 		}
 	}
 
Index: tests/phpunit/tests/db.php
===================================================================
--- tests/phpunit/tests/db.php	(revision 30587)
+++ tests/phpunit/tests/db.php	(working copy)
@@ -281,12 +281,13 @@
 
 		$current_modes = $wpdb->get_var( 'SELECT @@SESSION.sql_mode;' );
 
-		$new_modes = array( 'IGNORE_SPACE', 'NO_AUTO_CREATE_USER' );
+		$new_modes = $expected_modes = array( 'IGNORE_SPACE', 'NO_AUTO_CREATE_USER' );
+		$expected_modes[] = 'STRICT_ALL_TABLES';
 
 		$wpdb->set_sql_mode( $new_modes );
 
 		$check_new_modes = $wpdb->get_var( 'SELECT @@SESSION.sql_mode;' );
-		$this->assertEqualSets( $new_modes, explode( ',', $check_new_modes ) );
+		$this->assertEqualSets( $expected_modes, explode( ',', $check_new_modes ) );
 
 		$wpdb->set_sql_mode( explode( ',', $current_modes ) );
 	}
@@ -342,6 +343,146 @@
 	}
 
 	/**
+	 * @ticket 21212
+	 */
+	function test_set_sql_mode_strict() {
+		global $wpdb;
+		$wpdb->set_sql_mode();
+		$sql_modes = $wpdb->get_var( 'SELECT @@SESSION.sql_mode;' );
+		$this->assertContains( 'STRICT_ALL_TABLES', explode( ',', $sql_modes ) );
+	}
+
+	/**
+	 * @ticket 21212
+	 */
+	function test_strict_mode_numeric_strings() {
+		global $wpdb;
+		$post_id = $this->factory->post->create();
+		$wpdb->update( $wpdb->posts, array( 'post_parent' => 4 ), array( 'ID' => $post_id ), array( '%s' ) );
+		$this->assertContains( "`post_parent` = '4'", $wpdb->last_query );
+		$this->assertEmpty( $wpdb->last_error );
+	}
+
+	/**
+	 * @ticket 21212
+	 */
+	function test_strict_mode_numeric_strings_using_query() {
+		global $wpdb;
+		$post_id = $this->factory->post->create();
+		$wpdb->query( $wpdb->prepare( "UPDATE $wpdb->posts SET post_parent = %s WHERE ID = %s", '4', $post_id ) );
+		$this->assertEmpty( $wpdb->last_error );
+	}
+
+	/**
+	 * @ticket 21212
+	 */
+	function test_strict_mode_nan() {
+		global $wpdb;
+		$post_id = $this->factory->post->create();
+		$suppress = $wpdb->suppress_errors( true );
+		$wpdb->update( $wpdb->posts, array( 'post_parent' => 'foo' ), array( 'ID' => $post_id ), array( '%s' ) );
+		$this->assertContains( "`post_parent` = 'foo'", $wpdb->last_query );
+		$this->assertContains( 'Incorrect integer value', $wpdb->last_error );
+		$wpdb->suppress_errors( $suppress );
+	}
+
+	/**
+	 * @ticket 21212
+	 */
+	function test_strict_mode_nan_using_query() {
+		global $wpdb;
+		$post_id = $this->factory->post->create();
+		$suppress = $wpdb->suppress_errors( true );
+		$wpdb->query( $wpdb->prepare( "UPDATE $wpdb->posts SET post_parent = %s WHERE ID = %s", 'foo', $post_id ) );
+		$this->assertContains( 'Incorrect integer value', $wpdb->last_error );
+		$wpdb->suppress_errors( $suppress );
+	}
+
+	/**
+	 * @ticket 21212
+	 */
+	function test_strict_mode_number_start_of_string() {
+		global $wpdb;
+		$post_id = $this->factory->post->create();
+		$suppress = $wpdb->suppress_errors( true );
+		$wpdb->query( $wpdb->prepare( "UPDATE $wpdb->posts SET post_parent = %s WHERE ID = %s", '4foo', $post_id ) );
+		$this->assertContains( "Data truncated for column 'post_parent'", $wpdb->last_error );
+		$wpdb->suppress_errors( $suppress );
+	}
+
+	/**
+	 * @ticket 21212
+	 */
+	function test_strict_mode_booleans_true() {
+		global $wpdb;
+		$user_id = $this->factory->user->create();
+		$wpdb->query( "UPDATE $wpdb->users SET user_status = true WHERE ID = $user_id" );
+		$this->assertEmpty( $wpdb->last_error );
+		$user = get_userdata( $user_id );
+		$this->assertSame( '1', $user->user_status );
+	}
+
+	/**
+	 * @ticket 21212
+	 */
+	function test_strict_mode_booleans_false() {
+		global $wpdb;
+		$user_id = $this->factory->user->create();
+		$wpdb->query( "UPDATE $wpdb->users SET user_status = false WHERE ID = $user_id" );
+		$this->assertEmpty( $wpdb->last_error );
+		$user = get_userdata( $user_id );
+		$this->assertEquals( '0', $user->user_status );
+	}
+
+	/**
+	 * @ticket 21212
+	 */
+	function test_strict_mode_zero_date_is_valid() {
+		global $wpdb;
+		$user_id = $this->factory->user->create();
+		$wpdb->query( "UPDATE $wpdb->users SET user_registered = '0000-00-00' WHERE ID = $user_id" );
+		$this->assertEmpty( $wpdb->last_error );
+		$user = get_userdata( $user_id );
+		$this->assertEquals( '0000-00-00 00:00:00', $user->user_registered );
+	}
+
+	/**
+	 * @ticket 21212
+	 */
+	function test_strict_mode_zero_datetime_is_valid() {
+		global $wpdb;
+		$user_id = $this->factory->user->create();
+		$wpdb->query( "UPDATE $wpdb->users SET user_registered = '0000-00-00 00:00:00' WHERE ID = $user_id" );
+		$this->assertEmpty( $wpdb->last_error );
+		$user = get_userdata( $user_id );
+		$this->assertEquals( '0000-00-00 00:00:00', $user->user_registered );
+	}
+
+	/**
+	 * @ticket 21212
+	 */
+	function test_strict_mode_invalid_dates_are_invalid() {
+		global $wpdb;
+		$user_id = $this->factory->user->create();
+		$suppress = $wpdb->suppress_errors( true );
+		$wpdb->query( "UPDATE $wpdb->users SET user_registered = '2014-02-29 00:00:00' WHERE ID = $user_id" );
+		$this->assertContains( 'Incorrect datetime value', $wpdb->last_error );
+		$wpdb->suppress_errors( $suppress );
+	}
+
+	/**
+	 * @ticket 21212
+	 */
+	function test_strict_mode_nulls_are_invalid() {
+		global $wpdb;
+		$user_id = $this->factory->user->create();
+		$suppress = $wpdb->suppress_errors( true );
+		$wpdb->query( "UPDATE $wpdb->users SET user_nicename = NULL WHERE ID = $user_id" );
+		$this->assertContains( 'cannot be null', $wpdb->last_error );
+		$wpdb->suppress_errors( $suppress );
+	}
+
+	/**
 	 * @ticket 25604
 	 * @expectedIncorrectUsage wpdb::prepare
 	 */