SQL exercise set

The first question is ：on Back heel and and where The difference between

The table structure is as follows ：

CREATE TABLE `a` (
  `id` int(11) NOT NULL,
  `name` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
INSERT INTO `a` VALUES (1, 'a');
INSERT INTO `a` VALUES (2, 'b');
INSERT INTO `a` VALUES (3, 'c');

CREATE TABLE `b` (
  `id` int(11) NOT NULL,
  `name` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
INSERT INTO `b` VALUES (1, ' nail ');
INSERT INTO `b` VALUES (2, ' B ');

SQL1：SELECT a.*,b.* FROM a LEFT JOIN b on a.id = b.id; The execution result is ：

+----+------+------+------+
| id | name | id   | name |
+----+------+------+------+
|  1 | a    |    1 |  nail    |
|  2 | b    |    2 |  B    |
|  3 | c    | NULL | NULL |
+----+------+------+------+

SQL2：SELECT a.*,b.* FROM a LEFT JOIN b ON a.id = b.id AND b.id=2; The execution result is ：

+----+------+------+------+
| id | name | id   | name |
+----+------+------+------+
|  1 | a    | NULL | NULL |
|  2 | b    |    2 |  B    |
|  3 | c    | NULL | NULL |
+----+------+------+------+

SQL3：SELECT a.*,b.* FROM a LEFT JOIN b ON a.id = b.id WHERE b.id=2; The execution result is ：

+----+------+------+------+
| id | name | id   | name |
+----+------+------+------+
|  2 | b    |    2 |  B    |
+----+------+------+------+

Summary ：
1.on For later conditions and, Whether the conditions are true or not, all the data in the left table will be displayed .
2.on For later conditions where, stay left join Filter based on the generated table , At this time, I will put where Filter out those that are not established in .

The second question is ：…