高分求救，一个高难度的ＳＱＬ语句？？

[田九]

一个高难度的ＳＱＬ语句，不知道能不能实现。
如果表Ａ有符合条件的，就取Ａ的记录，如果没有就取表Ｂ的记录。
表Ａ和表Ｂ结构是一样的

[火龙岛主]

select * from a where exist (select 1 from a where 你的条件)
union
select * from b where exist (select 1 from b where 你的条件)

[田九]

这样写不行，如果该条件，都满足呢，不就都取出来了。

[田九]

因为有可能，两个表都满足的情况呀。

[hevin]

写了一个例子，你可以参考一下：

SQL> select * from a;

C1 C2
------- --------------------
100 Steven
101 Neena
102 Lex
103 Alexander
104 Bruce
105 David

6 rows selected

SQL> select * from b;

C1 C2
------- --------------------
103 Alexander
104 Bruce
105 David
106 Valli
107 Diana
108 Nancy
109 Daniel

7 rows selected

SQL>
SQL> SELECT *
2 FROM a
3 WHERE c1 > 101
4 UNION
5 SELECT *
6 FROM b
7 WHERE c1 > 101 AND NOT EXISTS (SELECT 1
8 FROM a
9 WHERE a.c1 > 101 AND rownum <=1);

C1 C2
------- --------------------
102 Lex
103 Alexander
104 Bruce
105 David

SQL>
SQL> SELECT *
2 FROM a
3 WHERE c1 > 106
4 UNION
5 SELECT *
6 FROM b
7 WHERE c1 > 106 AND NOT EXISTS (SELECT 1
8 FROM a
9 WHERE a.c1 > 106 AND rownum <=1);

C1 C2
------- --------------------
107 Diana
108 Nancy
109 Daniel

SQL>
SQL> SELECT *
2 FROM a
3 WHERE c1 = 101
4 UNION
5 SELECT *
6 FROM b
7 WHERE c1 =101 AND NOT EXISTS (SELECT 1
8 FROM a
9 WHERE a.c1 = 101 AND rownum <=1);

C1 C2
------- --------------------
101 Neena

SQL>
SQL> SELECT *
2 FROM a
3 WHERE c1 = 104
4 UNION
5 SELECT *
6 FROM b
7 WHERE c1 =104 AND NOT EXISTS (SELECT 1
8 FROM a
9 WHERE a.c1 = 104 AND rownum <=1);

C1 C2
------- --------------------
104 Bruce

SQL>
SQL> SELECT *
2 FROM a
3 WHERE c1 = 108
4 UNION
5 SELECT *
6 FROM b
7 WHERE c1 =108 AND NOT EXISTS (SELECT 1
8 FROM a
9 WHERE a.c1 = 108 AND rownum <=1);

C1 C2
------- --------------------
108 Nancy

SQL>

[kingkingkingking]

如果结构相同，数据量不是很大
用以下语句
select * from a1 where 条件
union
select * from a2 where 条件
两个语句的条件相同即可

[tongyu10068]

declare
v_temp number(10);
v_temp2 varchar(30);
begin
v_temp2 :=&输入条件;
select count(*) into v_temp from a where 列名 =v_temp2;
if (v_temp)!=0 then
select * from a where 列名 =v_temp2;
else
select * from b where 列名 =v_temp2;
end if;
end;

[Eric_1999]

select * from a where exist (select 1 from a where 你的条件)
union
select * from b where exist (select 1 from b where 你的条件)
and not exist (select 1 from a where 你的条件)

[wangzk0206]

select * from a where 条件
union
select * from b where 条件 and not exists (select 1 from a where 条件)

[小李木耳]

select * from a1 where 条件
union
select * from a2 where 条件 and (select count(*) from a1 where 条件)=0

[xueru9999]

我同意love_2008(love2008) 的写法