ID | Name
1 Brain
2 Amy
ID | WorkDay | MissionCode
1 2019-01-01 2360
1 2019-02-01 2470
2 2019-01-01 4470
2 2019-02-01 7210
What I want to achieve is, I want to get all table 1 fields and left join on the table 2 but only the latest WorkDay values, like this:
ID | Name | WorkDay | MissionCode
1 Brain 2019-02-01 2470
2 Amy 2019-02-01 7210
What I've tried so far is:
SELECT
table1.*, t2.WorkDay, t2.MissionCode
FROM
table1
LEFT JOIN
(SELECT
*
FROM
table2
ORDER BY
WorkDay DESC
LIMIT 0,1) AS t2
ON
t2.id = table1.id
But it returns NULL values from table2, like this:
ID | Name | WorkDay | MissionCode
1 Brain NULL NULL
2 Amy NULL NULL
I tested the same query adding extra WHERE clause to the inner join's select command and it succedeed.
SELECT
table1.*, t2.WorkDay, t2.MissionCode
FROM
table1
LEFT JOIN
(SELECT
*
FROM
table2
**WHERE id = 1**
ORDER BY
WorkDay DESC
LIMIT 0,1) AS t2
ON
t2.id = table1.id
And it returns ok, for the first row ofcourse:
ID | Name | WorkDay | MissionCode
1 Brain 2019-02-01 2470
2 Amy NULL NULL
But I can't use
WHERE id = table1.id
because MySQL says
Unknown column 'table1.id' in 'where clause'
So, what is the correct way of this ?
You can use a simple JOIN
to table2
, just put the MAX(WorkDay)
condition into the JOIN
condition as a correlated subquery, where you can access the table1
id value:
SELECT *
FROM table1 t1
JOIN table2 t2 ON t2.id = t1.id AND
t2.WorkDay = (SELECT MAX(WorkDay)
FROM table2
WHERE table2.id = t1.id)
Output:
ID Name ID WorkDay MissionCode
1 Brain 1 2019-02-01 2470
2 Amy 2 2019-02-01 7210