存在时的 DB2 案例

Question

正在搜索 table 以查看它是否包含 X 如果找不到 X 我需要它 return Y。这是我目前尝试过的方法：

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);

我得到 0 行 returned，其中找不到 A、B 或 C，而不是 Y。但如果它找到 A、B 或 C，它会正确 return十、

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

我发现这适用于 DB2：

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

也适用于 DB2 java 驱动程序。

Answer 3

这在 DB2 中用作记录存在性测试

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

存在时的 DB2 案例

DB2 Case When Exists

null

db2

case

exists