Getting first one depending on the current row - sql

Question

There are three tables, such as equip_type, output_history, and time_history in Oracle DB. How to join the three tables as shown below?
(DBMS: Oracle)
EQUIP MODEL DATE1 QUANTITY DATE2 TIME EQUIP_TYPE
---- ---- ---------- ------ -------- ---- ----------
e1 m1 20180103 10 20180101 6 A
e1 m1 20180106 20 20180105 5 A
Notice that at the point of DATE1 '20180103' in output_history, DATE2 '20180101' in time_history is the most recent one.
At the point of DATE1 '20180106' in output_history, 'DATE2 20180105' in time_history is the most recent one.
--equip_type table and the date
CREATE TABLE equip_type (
EQUIP_TYPE VARCHAR(60),
EQUIP VARCHAR(60)
);
INSERT INTO equip_type VALUES ('A','e1');
-- output_history and data
CREATE TABLE output_history (
EQUIP VARCHAR(60),
MODEL VARCHAR(60),
Data1 VARCHAR(60),
QUANTITY NUMBER(10)
);
INSERT INTO output_history VALUES ('e1','m1','20180103',10);
INSERT INTO output_history VALUES ('e1','m1','20180106',20);
--time_history table and data
CREATE TABLE time_history (
EQUIP VARCHAR(60),
MODEL VARCHAR(60),
Data2 VARCHAR(60),
time NUMBER(10)
);
INSERT INTO time_history VALUES ('e1','m1','20180101',6);
INSERT INTO time_history VALUES ('e1','m1','20180105',5);

Answer 1

You can use a correlated subquery with a NOT EXIST condition to select the closest related record in time_history.
I tested below query on MySQL in this db fiddle. You did not tag the RDBMS you are using. I tested on MySQL, but this is standard SQL that will work on most RDBMS.
SELECT
o.equip,
o.model,
o.data1,
o.quantity,
t.data2,
t.time,
e.equip_type
FROM
output_history o
INNER JOIN equip_type e ON e.equip = o.equip
INNER JOIN time_history t ON t.equip = o.equip AND t.data2 <= o.data1
WHERE NOT EXISTS (
SELECT 1
FROM time_history
WHERE
equip = o.equip
AND data2 <= o.data1
AND data2 > t.data2
)
Sid notes : the query will be always lookup the most recent time_history record before the current output_history record (even if there is a closest record in the future, it will not be selected)
Disclaimer : don't store dates as strings, this is a recipe for disaster. Use the relevant datatype according to your RDBMS. In your use case, it works only because dates are formated in a way that they can be easily sorted.