Question

Im trying to update a table with data from another table, the relation between the two tables ,
is a one-to-many relation. I have stubbed out a example here:

UPDATE
    foo f
INNER JOIN 
    bar b 
ON 
    b.fooId = f.Id 
SET
    f.something1 = b.barSome1,
    f.something2 = b.barSome1, 
    f.something3 =  b.barSome1, 
    f.something4 = b.barSome1

But my example only takes me half way,
What I want to do is to select the top 1 bar based on latest
bar.post_date or bar.Id or check if bar.barSome1 (lets say its a varchar) is null or empty.

As it is now it’s some internal ordering of my bar’s

Answer 1

So you’ll need to add a WHERE clause to limit which foo rows are modified.

UPDATE
    foo f
INNER JOIN 
    bar b 
ON 
    b.fooId = f.Id 
SET
    f.something1 = b.barSome1,
    f.something2 = b.barSome1, 
    f.something3 =  b.barSome1, 
    f.something4 = b.barSome1
WHERE 
  /* only the latest foo.Id */
  f.Id = (SELECT MAX(Id) FROM foo)
  /* Or f.barSome1 is NULL or an empty string */
  OR (f.barSome1 IS NULL OR f.barSome1 = '')

To do it by newest post_date use a construct like:

WHERE
  f.id = (SELECT id FROM (SELECT id, post_date FROM foo HAVING post_date = MAX(post_date)) AS maxpost)
  OR (f.barSome1 IS NULL OR f.barSome1 = '')

Update

To modify all rows of foo with matching criteria from a single row of bar, you can use a join against a subquery without an ON clause. Test this out first with a SELECT statement. I’ve never tried anything like this and accept no responsibility if it destroys your foo table. 🙂 Backup first.

Test the JOIN

SELECT f.*, barsub.*
FROM 
  foo f
  JOIN (SELECT * FROM bar WHERE Id = (SELECT MAX(Id) FROM bar)) barsub

The UPDATE should work the same way

UPDATE foo f
/* subquery returns one row so all rows of foo are joined to it */
JOIN (SELECT * FROM bar WHERE Id = (SELECT MAX(Id) FROM bar)) barsub
/* no ON clause */
SET
   f.something1 = barsub.barSome1,
   f.something2 = barsub.barSome1, 
   f.something3 = barsub.barSome1, 
   f.something4 = barsub.barSome1