什么是正确的SQL语法？

Question

I am struggling to resolve a SQL syntax problem in a Laravel Eloquent query. So I've gone right back to the root, and tried to get a query working in simple SQL. And I'm stumped. I'm no expert, but this is baffling me.

I've had a look at the Update with Join in SQLite post, but I think that one is more around the issue of joins.

I'm testing this on SQLLite, which surely is representative of SQL proper?

Two tables: Instructions and Knowns. I want to update one column in the Knowns from (the latest) Instructions. Get that right, and I can solve the rest myself (I hope!).

-- This works fine
Select instructions.rowid from instructions 
where instructions.EngagementTitle not null


-- This doesn't
UPDATE knowns 
SET    EngagementTitle = instructions.EngagementTitle
WHERE  id IN (
  SELECT knowns.id 
  FROM   knowns 
  INNER JOIN instructions 
  ON knowns.reference = instructions.reference
) 

Error Message

no such column: instructions.EngagementTitle: 

UPDATE knowns 
SET    EngagementTitle = instructions.EngagementTitle 
WHERE  id IN (
  SELECT knowns.id 
  FROM   knowns 
  LEFT JOIN instructions 
  ON knowns.reference = instructions.reference
) 

Both tables both have the column - triple checked.

`EngagementTitle` varchar NOT NULL

What am I missing?

Answer 1

UPDATE knowns 
SET    EngagementTitle = instructions.EngagementTitle 
...

The instructions table isn't "known" here, only knowns . The scope of the table expressions is "downwards" only. You can use a table introduced in a superquery in a subquery but not the other way round.

Try using subselects:

UPDATE knowns
       SET engagementtitle = (SELECT instructions.engagementtitle
                                     FROM instructions
                                     WHERE instructions.reference = knowns.reference)
       WHERE EXISTS (SELECT *
                            FROM instructions 
                            WHERE instructions.reference = knowns.reference);

I also replaced your WHERE with an EXISTS . I guess that is how you actually wanted it. It will only update rows from knowns where an entry exists in instructions for. The way you did it, with a LEFT JOIN in the subquery to IN , you'd just updated all rows, as a LEFT JOIN includes all rows from the left table, thus all IDs from knowns are in the result (You maybe wanted an INNER JOIN , that would have worked.).

But note, there must no more than one entry in instructions for one row in knowns for this to work. I silently assumed that this is the case.