不要让mysql犹豫不决

 1 DROP TABLE IF EXISTS `w1`;
 2 CREATE TABLE `w1` (
 3   `waid` int(11) NOT NULL AUTO_INCREMENT,
 4   `wa1` varchar(255) COLLATE utf8mb4_bin DEFAULT NULL,
 5   `wa2` varchar(255) COLLATE utf8mb4_bin DEFAULT NULL,
 6   PRIMARY KEY (`waid`)
 7 ) ENGINE=MyISAM AUTO_INCREMENT=3 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin;
 8
 9 -- ----------------------------
10 -- Records of w1
11 -- ----------------------------
12 INSERT INTO `w1` VALUES (‘1‘, ‘w1-1‘, ‘1‘);
13 INSERT INTO `w1` VALUES (‘2‘, ‘w1-2‘, ‘2‘);

 1 DROP TABLE IF EXISTS `w2`;
 2 CREATE TABLE `w2` (
 3   `wbid` int(11) NOT NULL AUTO_INCREMENT,
 4   `wb1` varchar(255) COLLATE utf8mb4_bin DEFAULT NULL,
 5   `wb2` varchar(255) COLLATE utf8mb4_bin DEFAULT NULL,
 6   PRIMARY KEY (`wbid`)
 7 ) ENGINE=MyISAM AUTO_INCREMENT=3 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin;
 8
 9 -- ----------------------------
10 -- Records of w2
11 -- ----------------------------
12 INSERT INTO `w2` VALUES (‘1‘, ‘w2-1‘, ‘2‘);
13 INSERT INTO `w2` VALUES (‘2‘, ‘w2-2‘, ‘2‘);

 1 SELECT * FROM w1 LEFT JOIN w2 ON w1.wa2=w2.wb2
 2 SELECT * FROM w1 LEFT JOIN w2 ON w2.wb2=w1.wa2
 3
 4 SELECT * FROM w2 RIGHT JOIN w1 ON w1.wa2=w2.wb2
 5 SELECT * FROM w2 RIGHT JOIN w1 ON w2.wb2=w1.wa2
 6
 7
 8 SELECT * FROM w2 LEFT JOIN w1 ON w1.wa2=w2.wb2
 9 SELECT * FROM w2 LEFT JOIN w1 ON w2.wb2=w1.wa2
10
11 SELECT * FROM w1 RIGHT JOIN w2 ON w1.wa2=w2.wb2
12 SELECT * FROM w1 RIGHT JOIN w2 ON w2.wb2=w1.wa2
13
14
15 SELECT * FROM w1 INNER JOIN w2 ON w1.wa2=w2.wb2
16 SELECT * FROM w1 INNER JOIN w2 ON w2.wb2=w1.wa2
17 SELECT * FROM w1 CROSS JOIN w2 ON w1.wa2=w2.wb2
18 SELECT * FROM w1 CROSS JOIN w2 ON w2.wb2=w1.wa2
19
20 SELECT * FROM w2 INNER JOIN w1 ON w1.wa2=w2.wb2
21 SELECT * FROM w2 INNER JOIN w1 ON w2.wb2=w1.wa2
22 SELECT * FROM w2 CROSS JOIN w1 ON w1.wa2=w2.wb2
23 SELECT * FROM w2 CROSS JOIN w1 ON w2.wb2=w1.wa2