使用选择语句中的数据选择子查询？

Question

I have two tables, headers and lines . I need to grab the batch_submission_date from the header table, but sometimes a query for batch_id will return a null for batch_submission_date, but will also return a parent_batch_id, and if we query THAT parent_batch_id as a batch_id, it will then return the correct batch_submission_date.

eg

SELECT t1.batch_id, 
    t1.parent_batch_id, 
    t2.batch_submission_date 
FROM db.headers t1, db.lines t2 
WHERE t1.batch_id = '12345';

output = 12345, 99999, null

Then we use that parent batch_id as a batch_id :

SELECT t1.batch_id, 
    t1.parent_batch_id, 
    t2.batch_submission_date
FROM db.headers t1, db.lines t2
WHERE t1.batch_id = '99999';

and we get output = 99999,99999,'2018-01-01'

So I'm trying to write a query that will do this for me - anytime a batch_id's batch_submission_date is null, we find that batch_id's parent batch_id and query that instead.

This was my idea - but I just get back null both for bp_batch_submission_date and for new_submission_date .

SELECT
    t1.parent_id as parent_id,
    t1.BATCH_ID as bp_batch_id,
    t2.BATCH_LINE_NUMBER as bp_batch_li,
    t1.BATCH_SUBMISSION_DATE as bp_batch_submission_date,

    CASE 
        WHEN t1.BATCH_SUBMISSION_DATE is null
    THEN
        (SELECT a.BATCH_SUBMISSION_DATE 
        FROM 
        db.headers a,
        db.lines b 
        WHERE 
            a.SD_BATCH_HEADERS_SKEY = b.SD_BATCH_HEADERS_SKEY   
            and a.parent_batch_id = bp_batch_id
            and b.batch_line_number = bp_batch_li
        ) END as new_submission_date

FROM    
    db.headers t1,
    db.lines t2 
WHERE
    t1.SD_BATCH_HEADERS_SKEY = t2.SD_BATCH_HEADERS_SKEY   
    and (t1.BATCH_ID = '12345' or t1.PARENT_BATCH_ID = '12345') 
    and t2.BATCH_LINE_NUMBER = '1'              
GROUP BY
    t2.BATCH_CLAIM_LINE_STATUS_DESC,
    t1.PARENT_BATCH_ID,
    t1.BATCH_ID,
    t2.BATCH_LINE_NUMBER,
    t1.BATCH_SUBMISSION_DATE;

is what I'm trying to do possible? using the bp_batch_id and bp_batch_li variables

Answer 1

Use CTE (common table expression) to avoid redundant code, then use coalesce() to find parent date in case of null. In your first queries you didn't attach joining condition between two tables, I assumed it's based on sd_batch_headers_skey like in last query.

^{dbfiddle demo}

with t as (
    select h.batch_id, h.parent_batch_id, l.batch_submission_date bs_date
      from headers h
      join lines l on l.sd_batch_headers_skey = h.sd_batch_headers_skey 
                  and l.batch_line_number = '1' )
select batch_id, parent_batch_id, 
       coalesce(bs_date, (select bs_date from t x where x.batch_id = t.parent_batch_id)) bs_date
  from t 
  where batch_id = 12345;

You could use simpler syntax with connect by and level <= 2 but if in your data there are really rows containing same ids ( 99999 , 99999 ) then we get cycle error.