Question

I’m coming across this situation alot, I’ll have a query that will have one table needed in a join condition that may have no entries therefore requiring me to use a LEFT JOIN. I can’t wrap my head around the syntax when it’s used with more than 1 join.

I’ll have:

SELECT A.*, B.*, C.*
FROM A, B, C
WHERE A.id = C.id
AND C.aid = A.id
AND B.cid = C.id

Along comes D with the possibility of being empty and I have to rewrite the query and run into problems.

How can I simply join D to any one of these tables?

Answer 1

My advice is to never specify more than one column on FROM clause.

For clarity, it’s better to always:

• Use JOIN clause
• Use aliases
• Specify columns of joined tables on left side of equal sign

Example:

SELECT a.*, b.*, c.*
FROM ATable a
INNER JOIN BTable b
  ON b.id = a.id
INNER JOIN CTable c
  ON c.id = a.id
WHERE a.someColumn = 'something'

Not sure about MySQL, but in some other SQL flavors, you can use the same on UPDATES and DELETES, like:

DELETE FROM a
FROM ATable a
INNER JOIN BTable b
  ON b.id = a.id
INNER JOIN CTable c
  ON c.id = a.id
WHERE a.someColumn = 'something'

or

UPDATE a
SET  something = newValue
FROM ATable a
INNER JOIN BTable b
  ON b.id = a.id
INNER JOIN CTable c
  ON c.id = a.id
WHERE a.someColumn = 'something'