如果在 db2 sql 查询中的上一个表中没有找到记录，则对下一个表执行列搜索

Question

I have three tables with same columns

table 1
    col1 col2 col3 col4

table 2
    col1 col2 col3 col4

table 3
    col1 col2 col3 col4

I have to perform a search if record is not found on table1 then only go to search in table2 and if not found in table2 then go to table3 . But if record found in any of these table then perform some calculation on col4 and return col4 without execution further. I am using DB2 but not able to find the exact solution. How can i achieve this?.

Answer 1

You can try this:

select col1 col2 col3 col4
from (
   select col1 col2 col3 col4, 1 as lvl
   from table_1
   where some_condition  

   union all

   select col1 col2 col3 col4, 2 as lvl
   from table_2
   where some_condition

   union all

   select col1 col2 col3 col4, 3 as lvl
   from table_3
   where some_condition) as t
order by lvl
limit 1 

Answer 2

If you want to keep this as one query, you can use UNION ALL to get the correct table:

SELECT col4, 1 as SortCol
    FROM Table1
    WHERE col1 = 'whatever'
UNION ALL
SELECT col4, 2 as SortCol
    FROM Table2
    WHERE col1 = 'whatever'
UNION ALL
SELECT col4, 3 as SortCol
    FROM Table3
    WHERE col1 = 'whatever'

ORDER BY SortCol
FETCH 1 ROW ONLY;

---

EDIT Another method is possible. I must say, I'm unsure coming from SQL Server the exact syntax, but it would be something like this:

SELECT COALESCE(t1.col4, t2.col4, t3.col4)

FROM (VALUES (@col1, @col2) ) v(col1, col2)
LEFT JOIN Table1 t1 ON t1.col1 = v.col1 AND t1.col2 = v.col2
LEFT JOIN Table2 t2 ON t2.col1 = v.col1 AND t2.col2 = v.col2
    AND t1.col4 IS NULL
LEFT JOIN Table3 t3 ON t3.col1 = v.col1 AND t3.col2 = v.col2
    AND t1.col4 IS NULL AND t2.col4 IS NULL;

The idea being to use the VALUES clause (or a SELECT with no FROM ) as a driving row.

Answer 3

Similar to what the other posters suggest, you could use code like this if you explicitly want to follow your "if then" logic

CREATE TABLE TABLE_1(C1 INT, C2 INT, C3 INT, C4 INT)

CREATE TABLE TABLE_2(C1 INT, C2 INT, C3 INT, C4 INT)

CREATE TABLE TABLE_3(C1 INT, C2 INT, C3 INT, C4 INT)

WITH
  C4(C1, C2, C3, C4) AS (VALUES (1,2,3,4))
, T1 AS ( SELECT '1' AS LVL, * FROM TABLE_1 JOIN C4 USING (C1, C2, C3, C4) )
, T2 AS ( SELECT '2' AS LVL, * FROM TABLE_2 JOIN C4 USING (C1, C2, C3, C4) WHERE NOT EXISTS (SELECT 1 FROM T1))
, T3 AS ( SELECT '3' AS LVL, * FROM TABLE_3 JOIN C4 USING (C1, C2, C3, C4) WHERE NOT EXISTS (SELECT 1 FROM T2))
, T4 AS ( SELECT '4' AS LVL, * FROM C4                                     WHERE NOT EXISTS (SELECT 1 FROM T3))
SELECT * FROM T1 UNION ALL
SELECT * FROM T2 UNION ALL
SELECT * FROM T3 UNION ALL
SELECT * FROM T4