CODEWITHSUNDEEP
postgresql
dwlamb
dwlamb

Reputation: 925

Delete using left outer join in Postgres

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.

Upvotes: 58

Views: 51734

Answers (4)

Joshua Burgner
Joshua Burgner

Reputation: 1205

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.

Note the self join on tv_episodes.id in the WHERE clause. This avoids the sub-query route provided above.

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;

Upvotes: 118

Meow
Meow

Reputation: 180

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;

Upvotes: 5

bf2020
bf2020

Reputation: 732

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.

Upvotes: 3

dwlamb
dwlamb

Reputation: 925

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.

Upvotes: 15

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