實例與答案

問題：表T1裏有a,b,c...N個字段，表T2裏有a,b,c三個字段，然後想在T1中"c"與表T2中"c"相同的情況下，從表T2中將a,b覆蓋表T1中的a,b,怎麽做？

實驗表：

create table T1(a int,b int,c int,d int,e int);
create table T2(a int,b int,c int);
insert into T1 values(1,2,3,4,5);
insert into T1 values(10,20,3,4,5);
insert into T1 values(10,20,4,40,50);
insert into T2 values(-1,-1,3);
insert into T2 values(-2,-2,4);
 

查看表：

SQL> select * from T1;

     A      B          C      D      E
---------- ---------- ---------- ---------- ----------
     1      2          3      4      5
    10     20          3      4      5
    10     20          4     40     50

SQL> select * from T2;

     A      B          C
---------- ---------- ----------
    -1     -1          3
    -2     -2          4

SQL> 
 

思路：
更新數據的基本語句

update T1 set a=?,b=? where ?

怎麽選出a呢？

SQL> select a.a from T2 a,T1 b where a.c=b.c;

     A
----------
    -1
    -1
    -2

SQL> 

同樣可以選出b

SQL> select a.b from T2 a,T1 b where a.c=b.c;

     B
----------
    -1
    -1
    -2

SQL> 

where是什麽？怎麽從集合中取出唯一的值？

SQL> update T1 set a=(select a from T2 where T1.c=T2.c),b=(select b from T2 where T1.c=T2.c) where T1.c in (select c from T2);

3 rows updated.

SQL> 
 

查看結果

SQL> select * from T1;

     A      B          C      D      E
---------- ---------- ---------- ---------- ----------
    -1     -1          3      4      5
    -1     -1          3      4      5
    -2     -2          4     40     50

SQL> 

oracle菜鳥學習之 復雜的更新語句使用