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table a(id, type):
id     type 
----------------------------------
1      1         
2      1          
3      2         
table b(id, class):
id    class 
---------------------------------
1      1
2      2
sqlÓï¾ä1£ºselect a.*, b.* from a left join b on a.id = b.id and a.type = 1;
sqlÓï¾ä2£ºselect a.*, b.* from a left join b on a.id = b.id where a.type = 1;
sqlÓï¾ä3£ºselect a.*, b.* from a left join b on a.id = b.id and b.class = 1;
sqlÓï¾ä1µÄÖ´Ðнá¹ûΪ£º
a.id    a.type    b.id    b.class
----------------------------------------
1        1            1        1
2        1            2        2
3        2               
sqlÓï¾ä2µÄÖ´Ðнá¹ûΪ£º
a.id    a.type    b.id    b.class
----------------------------------------
1        1            1        1
2        1            2        2
sqlÓï¾ä3µÄÖ´Ðнá¹ûΪ£º
a.id    a.type    b.id    b.class
----------------------------------------
1   &n