不要让mysql犹豫不决

 DROP TABLE IF EXISTS `w1`;
 CREATE TABLE `w1` (
   `waid` int(11) NOT NULL AUTO_INCREMENT,
   `wa1` varchar(255) COLLATE utf8mb4_bin DEFAULT NULL,
   `wa2` varchar(255) COLLATE utf8mb4_bin DEFAULT NULL,
   PRIMARY KEY (`waid`)
 ) ENGINE=MyISAM AUTO_INCREMENT=3 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin;

 -- ----------------------------
 -- Records of w1
 -- ----------------------------
 INSERT INTO `w1` VALUES ('', 'w1-1', '');
 INSERT INTO `w1` VALUES ('', 'w1-2', '');

 DROP TABLE IF EXISTS `w2`;
 CREATE TABLE `w2` (
   `wbid` int(11) NOT NULL AUTO_INCREMENT,
   `wb1` varchar(255) COLLATE utf8mb4_bin DEFAULT NULL,
   `wb2` varchar(255) COLLATE utf8mb4_bin DEFAULT NULL,
   PRIMARY KEY (`wbid`)
 ) ENGINE=MyISAM AUTO_INCREMENT=3 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin;

 -- ----------------------------
 -- Records of w2
 -- ----------------------------
 INSERT INTO `w2` VALUES ('', 'w2-1', '');
 INSERT INTO `w2` VALUES ('', 'w2-2', '');

 SELECT * FROM w1 LEFT JOIN w2 ON w1.wa2=w2.wb2
 SELECT * FROM w1 LEFT JOIN w2 ON w2.wb2=w1.wa2

 SELECT * FROM w2 RIGHT JOIN w1 ON w1.wa2=w2.wb2
 SELECT * FROM w2 RIGHT JOIN w1 ON w2.wb2=w1.wa2

 SELECT * FROM w2 LEFT JOIN w1 ON w1.wa2=w2.wb2
 SELECT * FROM w2 LEFT JOIN w1 ON w2.wb2=w1.wa2

 SELECT * FROM w1 RIGHT JOIN w2 ON w1.wa2=w2.wb2
 SELECT * FROM w1 RIGHT JOIN w2 ON w2.wb2=w1.wa2

 SELECT * FROM w1 INNER JOIN w2 ON w1.wa2=w2.wb2
 SELECT * FROM w1 INNER JOIN w2 ON w2.wb2=w1.wa2
 SELECT * FROM w1 CROSS JOIN w2 ON w1.wa2=w2.wb2
 SELECT * FROM w1 CROSS JOIN w2 ON w2.wb2=w1.wa2

 SELECT * FROM w2 INNER JOIN w1 ON w1.wa2=w2.wb2
 SELECT * FROM w2 INNER JOIN w1 ON w2.wb2=w1.wa2
 SELECT * FROM w2 CROSS JOIN w1 ON w1.wa2=w2.wb2
 SELECT * FROM w2 CROSS JOIN w1 ON w2.wb2=w1.wa2