Delete using left outer join in Postgres - postgresql

Question

I am switching a database from MySQL to Postgres SQL. A select query that worked in MySQL works in Postgres but a similar delete query does not.
I have two tables of data which list where certain back-up files are located. Existing data (ed) and new data (nd). This syntax will pick out existing data which might state where a file is located in the existing data table, matching it against equal filename and path, but no information as to where it is located in the new data:
SELECT ed.id, ed.file_name, ed.cd_name, ed.path, nd.cd_name
FROM tv_episodes AS ed
LEFT OUTER JOIN data AS nd ON
ed.file_name = nd.file_name AND
ed.path = nd.path
WHERE ed.cd_name = 'MediaLibraryDrive' AND nd.cd_name IS NULL;
I wish to run a delete query using this syntax:
DELETE ed
FROM tv_episodes AS ed
LEFT OUTER JOIN data AS nd ON
ed.file_name = nd.file_name AND
ed.path = nd.path
WHERE ed.cd_name = 'MediaLibraryDrive' AND nd.cd_name IS NULL;
I have tried DELETE ed and DELETE ed.* both of which render syntax error at or near "ed". Similar errors if I try without the alias of ed. If I attempt
DELETE FROM tv_episodes AS ed
LEFT JOIN data AS nd.....
Postgres sends back syntax error at or near "LEFT".
I'm stumped and can't find much on delete queries using joins specific to psql.

Answer 1

As others have noted, you can't LEFT JOIN directly in a DELETE statement. You can, however, self join on a primary key to the target table with a USING statement, then left join against that self-joined table.
DELETE FROM tv_episodes
USING tv_episodes AS ed
LEFT OUTER JOIN data AS nd ON
ed.file_name = nd.file_name AND
ed.path = nd.path
WHERE
tv_episodes.id = ed.id AND
ed.cd_name = 'MediaLibraryDrive' AND nd.cd_name IS NULL;
Note the self join on tv_episodes.id in the WHERE clause. This avoids the sub-query route provided above.

Answer 2

As bf2020 points out, postgres does not support JOINs when conducting a DELETE query. The proposed solution of a sub-query made me think of the solution. Refine the SELECT query from above and employ it as a sub-query to a DELETE query statement:
DELETE FROM tv_episodes
WHERE id in (
SELECT ed.id
FROM tv_episodes AS ed
LEFT OUTER JOIN data AS nd ON
ed.file_name = nd.file_name AND
ed.path = nd.path
WHERE ed.cd_name = 'MediaLibraryDrive' AND nd.cd_name IS NULL
);
Sub-queries can often be inefficient consuming time and CPU resources with some database systems, especially MySQL. From my experience I try to avoid using a sub-query due to that inefficiency plus that such queries are sometimes an easy way out to honing one's skill like learning JOIN syntax.
Since postgre does not permit delete queries using join, the above is the solution that works.

Answer 3

Use the DELETE... USING syntax:
DELETE FROM tv_episodes USING data WHERE
tv_episodes.file_name = data.file_name AND
tv_episodes.path = data.path AND
tv_episodes.cd_name = 'MediaLibraryDrive' AND
data.cd_name IS NULL;

Answer 4

Instead of
DELETE ed
FROM tv_episodes AS ed
LEFT OUTER JOIN data AS nd ON
ed.file_name = nd.file_name AND
ed.path = nd.path
WHERE ed.cd_name = 'MediaLibraryDrive' AND nd.cd_name IS NULL;
please try
DELETE FROM tv_episodes
WHERE cd_name = 'MediaLibraryDrive' AND
(tv_episodes.filename, tv_episodes.path IN
(SELECT ed.filename,
ed.path
FROM tv_episodes AS ed
INNER JOIN data AS nd
ON ed.file_name = nd.file_name
AND ed.path = nd.path
WHERE nd.cd_name IS NULL)
)
;
JOIN is not valid in a DELETE query according to the postgresql documentation. You might need to concatenate the left and right parts of the IN expression.