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liprais 问得好,只专注DCL语句了,对DML语句不重视,但是试一下就知道了
SQL> select * from a;
C1 C2
---------- ----------
1 1
2 2
3 3
SQL> select * from b;
C1 C2
---------- ----------
1 1
2 3
3 4
SQL> select * from a left join b on a.c1=b.c1 and a.c2=b.c2;
C1 C2 C1 C2
---------- ---------- ---------- ----------
1 1 1 1
2 2
3 3
SQL> select * from a left join b on a.c1=b.c1 where a.c2=b.c2;
C1 C2 C1 C2
---------- ---------- ---------- ----------
1 1 1 1
关键是要理解left join on和where