left join中關於where和on條件的幾個知識點：
    1.多表left join是會生成一張臨時表，並返回給用戶
    2.where條件是針對最後生成的這張臨時表進行過濾，過濾掉不符合where條件的記錄，是真正的不符合就過濾掉。
    3.on條件是對left join的右表進行條件過濾，但依然返回左表的所有行，右表中沒有的補為NULL
    4.on條件中如果有對左表的限制條件，無論條件真假，依然返回左表的所有行,但是會影響右表的匹配值。也就是說on中左表的限制條件只影響右表的匹配內容，不影響返回行數。
結論：
    1.where條件中對左表限制，不能放到on後面
    2.where條件中對右表限制，放到on後面，會有數據行數差異，比原來行數要多

測試：
創建兩張表：
CREATE TABLE t1(id INT,name VARCHAR(20));
insert  into `t1`(`id`,`name`) values (1,‘a11‘);
insert  into `t1`(`id`,`name`) values (2,‘a22‘);
insert  into `t1`(`id`,`name`) values (3,‘a33‘);
insert  into `t1`(`id`,`name`) values (4,‘a44‘);

CREATE TABLE t2(id INT,local VARCHAR(20));
insert  into `t2`(`id`,`local`) values (1,‘beijing‘);
insert  into `t2`(`id`,`local`) values (2,‘shanghai‘);
insert  into `t2`(`id`,`local`) values (5,‘chongqing‘);
insert  into `t2`(`id`,`local`) values (6,‘tianjin‘);

測試01：返回左表所有行，右表符合on條件的原樣匹配，不滿足條件的補NULL
[email protected]:cuigl 11:04:25 >SELECT t1.id,t1.name,t2.local FROM t1 LEFT JOIN t2 ON t1.id=t2.id;
+------+------+----------+
| id   | name | local    |
+------+------+----------+
|    1 | a11  | beijing  |
|    2 | a22  | shanghai |
|    3 | a33  | NULL     |
|    4 | a44  | NULL     |
+------+------+----------+
4 rows in set (0.00 sec)

測試02：on後面增加對右表的限制條件：t2.local=‘beijing‘
結論02：左表記錄全部返回，右表篩選條件生效
[email protected]:cuigl 11:19:42 >SELECT t1.id,t1.name,t2.local FROM t1 LEFT JOIN t2 ON t1.id=t2.id and t2.local=‘beijing‘;
+------+------+---------+
| id   | name | local   |
+------+------+---------+
|    1 | a11  | beijing |
|    2 | a22  | NULL    |
|    3 | a33  | NULL    |
|    4 | a44  | NULL    |
+------+------+---------+
4 rows in set (0.00 sec)

測試03：只在where後面增加對右表的限制條件：t2.local=‘beijing‘
結論03：針對右表，相同條件，在where後面是對最後的臨時表進行記錄篩選，行數可能會減少；在on後面是作為匹配條件進行篩選，篩選的是右表的內容。
[email protected]:cuigl 11:20:07 >SELECT t1.id,t1.name,t2.local FROM t1 LEFT JOIN t2 ON t1.id=t2.id where t2.local=‘beijing‘;   
+------+------+---------+
| id   | name | local   |
+------+------+---------+
|    1 | a11  | beijing |
+------+------+---------+
1 row in set (0.01 sec)

測試04：t1.name=‘a11‘ 或者 t1.name=‘a33‘
結論04：on中對左表的限制條件，不影響返回的行數，只影響右表的匹配內容
[email protected]:cuigl 11:24:46 >SELECT t1.id,t1.name,t2.local FROM t1 LEFT JOIN t2 ON t1.id=t2.id and t1.name=‘a11‘;  
+------+------+---------+
| id   | name | local   |
+------+------+---------+
|    1 | a11  | beijing |
|    2 | a22  | NULL    |
|    3 | a33  | NULL    |
|    4 | a44  | NULL    |
+------+------+---------+
4 rows in set (0.00 sec)
[email protected]:cuigl 11:25:04 >SELECT t1.id,t1.name,t2.local FROM t1 LEFT JOIN t2 ON t1.id=t2.id and t1.name=‘a33‘;
+------+------+-------+
| id   | name | local |
+------+------+-------+
|    1 | a11  | NULL  |
|    2 | a22  | NULL  |
|    3 | a33  | NULL  |
|    4 | a44  | NULL  |
+------+------+-------+
4 rows in set (0.00 sec)

測試05：where t1.name=‘a33‘ 或者 where t1.name=‘a22‘
結論05：where條件是在最後臨時表的基礎上進行篩選，顯示只符合最後where條件的行
[email protected]:cuigl 11:25:15 >SELECT t1.id,t1.name,t2.local FROM t1 LEFT JOIN t2 ON t1.id=t2.id where t1.name=‘a33‘;   
+------+------+-------+
| id   | name | local |
+------+------+-------+
|    3 | a33  | NULL  |
+------+------+-------+
1 row in set (0.00 sec)
[email protected]:cuigl 11:27:27 >SELECT t1.id,t1.name,t2.local FROM t1 LEFT JOIN t2 ON t1.id=t2.id where t1.name=‘a22‘;
+------+------+----------+
| id   | name | local    |
+------+------+----------+
|    2 | a22  | shanghai |
+------+------+----------+
1 row in set (0.00 sec)
 

mysql left join中where和on條件的區別