连接 MySQL 中的两个表，结果集中只有一行，从第二个表中选择了几列

Question

I am having a challenge to achieve the following

Have 2 tables namely Transaction Table and Transaction_Event_LOG_INFO table

Transaction Table Details

SL	TXN_ID	TXN_FOR	TXN_TYPE
1	111	Shopping	Debit
2	112	Rent	Debit

Transaction_Event_LOG_INFO table

SL	TXN_ID	EVT_ID	EVT_CODE	EVT_CREATED_DT
1	111	200	100	11-12-2020 12:00:00
1	111	201	101	11-12-2020 12:01:00
1	111	202	102	11-12-2020 12:02:00
1	111	203	103	11-12-2020 12:03:00
1	111	204	104	11-12-2020 12:04:00
1	112	205	100	11-12-2020 12:05:00
1	112	206	101	11-12-2020 12:06:00
1	112	207	102	11-12-2020 12:07:00
1	112	208	103	11-12-2020 12:08:00
1	112	209	104	11-12-2020 12:09:00

I need to join the above two tables based on TXN_ID (which is unique) and have just one row for every TXN ID and having all columns from 1st table and just the EVT_CREATED_DT from 2nd table like where EVT_CODE=100 and 104

like below

SL	TXN_ID	TXN_FOR	TXN_TYPE	EVT_CREATED_DT_FOR_100	EVT_CREATED_DT_104
1	111	Shopping	Debit	11-12-2020 12:00:00	11-12-2020 12:04:00
2	112	Rent	Debit	11-12-2020 12:05:00	11-12-2020 12:09:00

Answer 1

You can do cnoditional aggregation:

select t.*, te.evt_created_dt_100, te.evt_created_dt_104
from transaction t
inner join (
    select txn_id
        max(case when evt_code = 100 then evt_created_dt end) as evt_created_dt_100,
        max(case when evt_code = 104 then evt_created_dt end) as evt_created_dt_104
    from transaction_event_log_info
    where evt_code in (100, 104)
    group by txn_id
) te on te.txn_id = t.txn_id

In very recent versions of MySQL, this would be more efficiently phrased with a lateral join:

select t.*, te.*
from transaction t
cross join lateral (
    select
        max(case when te.evt_code = 100 then te.evt_created_dt end) as evt_created_dt_100,
        max(case when te.evt_code = 104 then te.evt_created_dt end) as evt_created_dt_104
    from transaction_event_log_info te
    where te.evt_code in (100, 104) and te.txn_id = t.txn_id
) te

If you want to allow transactions that have none of the two events, you would change the inner join to a left join - and the cross join lateral (...) te to a left join lateral (...) te on true .