根据列值，查询不同的表

Question

I have table A where each IDENTIFIER value is unique. Depending on whether its CC or DC, I need to join it with different tables, for example if its "CC_" I need to map it to table B and if its "DC_" I need to map it to table C.

TABLE A:

ID | IDENTIFIER
1  | CC_1234567
2  | DC_9494949

TABLE B:

ID| SOURCE.  |INSTRUMENT
7 | 1234567. | 545454

TABLE C:

ID  |  SOURCE  | INSTRUMENT
8   |. 9494949 |. 63636363

I want the result to look like this:

IDENTIFIER | INSTRUMENT
CC_1234567 | 545454
DC_9494949 | 63636363

First of all, how could I just get "1234567" from "CC_1234567" and second when writing a join query, I would get other columns in the final resulting table, such as B.INSTRUMENT, C.INSTRUMENT. Each IDENTIFIER in table A only maps to either table B OR table C.

Select A.IDENTIFIER, B.INSTRUMENT, C.INSTRUMENT 
from table.A 
inner join table.A on A.IDENTIFIER=B.INSTRUMENT AND A.IDENTIFIER=C.INSTRUMENT

Answer 1

You can left join twice, and then use coalesce() in the select clause:

select a.identifier, coalesce(b.instrument, c.instrument) instrument
from tablea
left join tableb b on a.identifier = concat('CC_', b.source)
left join tablec c on a.identifier = concat('DC_', c.source)

If there are matches in both tables, the above query gives priority to tableb .

If you want to exclude rows that have no match in either tables, then you can add the following where clause:

where a.source is not null or b.source is not null

Answer 2

You can use UNION like the following:

SELECT 
    A.IDENTIFIER,
    B.INSTRUMENT  
FROM
    A 
    INNER JOIN B 
        ON B.SOURCE = SUBSTR(A.IDENTIFIER, 4) 
WHERE A.IDENTIFIER LIKE 'CC_' 
UNION
SELECT 
    A.IDENTIFIER,
    C.INSTRUMENT 
FROM
    A 
    INNER JOIN C 
        ON C.SOURCE = SUBSTR(A.IDENTIFIER, 4) 
WHERE A.IDENTIFIER LIKE 'DC_' ;

Depending on "CC_" or "DC_", you join a different table and merge using UNION.

Answer 3

Try this: the right function combined with length should give you a handy way to strip these going forward.

SELECT 
   a.identifier,
   coalesce(b.instrument, c.instrument) AS instrument
FROM
     a LEFT JOIN b ON 
   right(a.identifier,length(a.identifier)-3) = b.source 
      LEFT JOIN c ON
   right(a.identifier,length(a.identifier)-3) = c.source;