存在时DB2情况

Question

搜索表格以查看它是否包含x，如果它找不到x，我需要返回y。这是我到目前为止尝试过的：

SELECT WHEN T1.X IS NULL THEN Y ELSE T1.X END AS XY 
FROM TABLE1 T1
LEFT JOIN TABLE2 T2 ON T1.X = T2.X
WHERE T1.X IN (A, B, C);
SELECT CASE WHEN T1.X IS NOT NULL THEN T1.X ELSE Y END AS XY
FROM TABLE1 T1
WHERE T1.X IN (A, B, C)
AND T1.X IN (
    SELECT T2.X
    FROM TABLE2 T2);
SELECT CASE WHEN EXISTS (SELECT X FROM TABLE1) THEN X ELSE Y END AS XY
FROM TABLE1 
WHERE X IN (A, B, C);

我将获得0行返回，它在找不到A，B或C，而不是Y。但是，如果它找到A，B或C，它确实返回了X。

Answer 1

简单联合

SELECT X FROM TABLE1 WHERE X IN (A, B, C) 
UNION 
SELECT Y FROM SYSIBM.SYSDUMMY1 
WHERE NOT EXISTS (SELECT X FROM TABLE1 WHERE X IN (A, B, C))

Answer 2

这是DB2

中的记录存在测试

SELECT
  CASE COUNT(*)
    WHEN 0 THEN 'FALSE'
    ELSE 'TRUE'
  END as IsRecordThere
FROM MYTABLE 
WHERE COL1 = 'whatever' AND COL2 = 'etc'

Answer 3

我发现它可以与db2合作：

SELECT A, B, 
Case C
WHEN 'Citron' THEN 'Lemon'
WHEN 'Charlie' THEN 'Name'
end as Choice, 
D
FROM TABLE with UR;

也与DB2 Java驱动程序一起使用。