mn_type_sql;
	}

	/**
	 * Adds column options.
	 *
	 * @param string $type    The native type.
	 * @param array  $options The options.
	 *
	 * @return string The SQL statement for the column options.
	 *
	 * @throws Exception If invalid arguments are supplied.
	 */
	public function add_column_options( $type, $options ) {
		$sql = '';
		if ( ! \is_array( $options ) ) {
			return $sql;
		}
		if ( \array_key_exists( 'unsigned', $options ) && $options['unsigned'] === true ) {
			$sql .= ' UNSIGNED';
		}
		if ( \array_key_exists( 'character', $options ) ) {
			$sql .= \sprintf( ' CHARACTER SET %s', $this->identifier( $options['character'] ) );
		}
		if ( \array_key_exists( 'collate', $options ) ) {
			$sql .= \sprintf( ' COLLATE %s', $this->identifier( $options['collate'] ) );
		}
		if ( \array_key_exists( 'auto_increment', $options ) && $options['auto_increment'] === true ) {
			$sql .= ' auto_increment';
		}
		if ( \array_key_exists( 'default', $options ) && $options['default'] !== null ) {
			if ( $this->is_sql_method_call( $options['default'] ) ) {
				throw new Exception( 'MySQL does not support function calls as default values, constants only.' );
			}
			if ( \is_int( $options['default'] ) ) {
				$default_format = '%d';
			}
			elseif ( \is_bool( $options['default'] ) ) {
				$default_format = "'%d'";
			}
			elseif ( $options['default'] === 'CURRENT_TIMESTAMP' ) {
				$default_format = '%s';
			}
			else {
				$default_format = "'%s'";
			}
			$default_value = \sprintf( $default_format, $options['default'] );
			$sql          .= \sprintf( ' DEFAULT %s', $default_value );
		}
		if ( \array_key_exists( 'null', $options ) ) {
			if ( $options['null'] === false || $options['null'] === 'NO' ) {
				$sql .= ' NOT NULL';
			}
			elseif ( $type === 'timestamp' ) {
				$sql .= ' NULL';
			}
		}
		if ( \array_key_exists( 'comment', $options ) ) {
			$sql .= \sprintf( " COMMENT '%s'", $this->quote_string( $options['comment'] ) );
		}
		if ( \array_key_exists( 'extra', $options ) ) {
			$sql .= \sprintf( ' %s', $this->quote_string( $options['extra'] ) );
		}
		if ( \array_key_exists( 'after', $options ) ) {
			$sql .= \sprintf( ' AFTER %s', $this->identifier( $options['after'] ) );
		}

		return $sql;
	}

	/**
	 * Returns a list of all versions that have been migrated.
	 *
	 * @return string[] The version numbers that have been migrated.
	 */
	public function get_migrated_versions() {
		$result = $this->select_all( \sprintf( 'SELECT version FROM %s', $this->get_schema_version_table_name() ) );
		return \array_column( $result, 'version' );
	}

	/**
	 * Adds a migrated version.
	 *
	 * @param string $version The version.
	 *
	 * @return bool Whether or not the version was succesfully set.
	 */
	public function add_version( $version ) {
		$sql = \sprintf( "INSERT INTO %s (version) VALUES ('%s')", $this->get_schema_version_table_name(), $version );

		return $this->execute_ddl( $sql );
	}

	/**
	 * Removes a migrated version.
	 *
	 * @param string $version The version.
	 *
	 * @return bool Whether or not the version was succesfully removed.
	 */
	public function remove_version( $version ) {
		$sql = \sprintf( "DELETE FROM %s WHERE version = '%s'", $this->get_schema_version_table_name(), $version );

		return $this->execute_ddl( $sql );
	}

	/**
	 * Returns a message displaying the current version
	 *
	 * @return string
	 */
	public function __toString() {
		return self::class . ', version ' . $this->version;
	}

	/**
	 * Returns an index name.
	 *
	 * @param string $table_name  The table name.
	 * @param string $column_name The column name.
	 *
	 * @return string The index name.
	 */
	private function get_index_name( $table_name, $column_name ) {
		$name = \preg_replace( '/\\W/', '_', $table_name );
		$name = \preg_replace( '/\\_{2,}/', '_', $name );
		// If the column parameter is an array then the user wants to create a multi-column index.
		if ( \is_array( $column_name ) ) {
			$column_str = \implode( '_and_', $column_name );
		}
		else {
			$column_str = $column_name;
		}
		$name .= \sprintf( '_%s', $column_str );
		return $name;
	}

	/**
	 * Returns the type of a query.
	 *
	 * @param string $query The query to run.
	 *
	 * @return int The query type.
	 */
	private function determine_query_type( $query ) {
		$query = \strtolower( \trim( $query ) );
		$match = [];
		\preg_match( '/^(\\w)*/i', $query, $match );
		$type = $match[0];
		switch ( $type ) {
			case 'select':
				return Constants::SQL_SELECT;
			case 'update':
				return Constants::SQL_UPDATE;
			case 'delete':
				return Constants::SQL_DELETE;
			case 'insert':
				return Constants::SQL_INSERT;
			case 'alter':
				return Constants::SQL_ALTER;
			case 'drop':
				return Constants::SQL_DROP;
			case 'create':
				return Constants::SQL_CREATE;
			case 'show':
				return Constants::SQL_SHOW;
			case 'rename':
				return Constants::SQL_RENAME;
			case 'set':
				return Constants::SQL_SET;
			default:
				return Constants::SQL_UNKNOWN_QUERY_TYPE;
		}
	}

	/**
	 * Detect whether or not the string represents a function call and if so
	 * do not wrap it in single-quotes, otherwise do wrap in single quotes.
	 *
	 * @param string $text The string.
	 *
	 * @return bool Whether or not it's a SQL function call.
	 */
	private function is_sql_method_call( $text ) {
		$text = \trim( $text );
		if ( \substr( $text, -2, 2 ) === '()' ) {
			return true;
		}
		return false;
	}

	/**
	 * Checks if a transaction is active.
	 *
	 * @return bool
	 */
	private function in_transaction() {
		return $this->in_transaction;
	}

	/**
	 * Starts a transaction.
	 *
	 * @return void
	 *
	 * @throws Exception If a transaction was already started.
	 */
	private function begin_transaction() {
		global $wpdb;

		if ( $this->in_transaction === true ) {
			throw new Exception( 'Transaction already started' );
		}
		$wpdb->query( 'START TRANSACTION' );
		$this->in_transaction = true;
	}

	/**
	 * Commits a transaction.
	 *
	 * @return void
	 *
	 * @throws Exception If no transaction was strated.
	 */
	private function commit() {
		global $wpdb;

		if ( $this->in_transaction === false ) {
			throw new Exception( 'Transaction not started' );
		}
		$wpdb->query( 'COMMIT' );
		$this->in_transaction = false;
	}

	/**
	 * Rollbacks a transaction.
	 *
	 * @return void
	 *
	 * @throws Exception If no transaction was started.
	 */
	private function rollback() {
		global $wpdb;

		if ( $this->in_transaction === false ) {
			throw new Exception( 'Transaction not started' );
		}
		$wpdb->query( 'ROLLBACK' );
		$this->in_transaction = false;
	}
}