Why does this SQLite UPDATE statement write changes to all records in the table? - sqlite

Question

This query returns 40319 records:
SELECT artist,
title,
composer AS alib_composer,
master.composer AS master_composer
FROM alib
INNER JOIN
master ON hex(alib.__path) = hex(master.__path) AND
alib.l_title = lower(master.title) AND
alib.composer != master.composer AND
length(alib.composer) != length(master.composer) AND
master.composer LIKE "% %" AND
alib.composer NOT LIKE "% %";
Translating above into the following UPDATE query results in all records in alib being updated. Could someone help me understand why this is the case?
UPDATE alib
SET composer = (
SELECT composer
FROM master
WHERE hex(alib.__path) = hex(master.__path) AND
alib.l_title = lower(master.title) AND
alib.composer != master.composer AND
length(alib.composer) != length(master.composer) AND
master.composer LIKE "% %" AND
alib.composer NOT LIKE "% %"
);
I suspect I know why, but turning the update into something like this:
UPDATE alib
SET composer = master.composer
WHERE hex(alib.__path) = hex(master.__path) AND
alib.l_title = lower(master.title) AND
alib.composer != master.composer AND
length(alib.composer) != length(master.composer) AND
master.composer LIKE "% %" AND
alib.composer NOT LIKE "% %";
causes SQLite to complain:
Error while executing SQL query on database 'x': no such column: master.composer

Answer 1

Because there is no WHERE clause on the outer/main UPDATE SQL i.e. it is basiclly
UPDATE alib SET composer = the_result_of_the_subquery;
i.e. UPDATE every row.
You want a WHERE clause to say which rows to update along the lines of :-
UPDATE alib SET composer = the_result_of_the_subquery WHERE ??????;
Or to use your code :-
UPDATE alib
SET composer = (
SELECT composer
FROM master
WHERE hex(alib.__path) = hex(master.__path) AND
alib.l_title = lower(master.title) AND
alib.composer != master.composer AND
length(alib.composer) != length(master.composer) AND
master.composer LIKE "% %" AND
alib.composer NOT LIKE "% %"
)
WHERE the_expression_to_restrict_which_rows_are_updated
;
This hasn't been fully tested but this may be solution you are looking for :-
WITH cte AS
(
SELECT
master.composer AS master_composer,
alib.rowid AS arowid
FROM alib
INNER JOIN
master ON hex(alib.__path) = hex(master.__path)
AND alib.l_title = lower(master.title)
AND alib.composer != master.composer
AND length(alib.composer) != length(master.composer)
AND master.composer LIKE '% %'
AND alib.composer NOT LIKE '% %'
)
UPDATE alib
SET composer =
(
SELECT cte.master_composer FROM cte WHERE cte.arowid = alib.rowid
)
WHERE alib.rowid IN (SELECT arowid FROM cte);
this uses a Common Table Expresssion(CTE) (temporay table that exists just for the query).
It utlises the rowid (a hidden column that every table has other than WITHOUT ROWID tables).
That is the CTE is based upon your original query that extracts the rows from alib that need to be updated. It only needs the composer from the master table and the rowid of the alib table for the update.
The UPDATE sets the composer to the value of the master_composer column of the respective row from the CTE according to the rowid of the row being updated.
The UPDATEs are only applied to rows that have a matching row in the CTE.
Testing (limited)
The following SQL was used for testing :-
DROP TABLE IF EXISTS alib;
DROP TABLE IF EXISTS master;
CREATE TABLE IF NOT EXISTS alib (__path,l_title,composer);
CREATE TABLE IF NOT EXISTS master (__path,title,artist,composer);
INSERT INTO alib VALUES
('path1','this','bert'),
('path2','something else','Joe'),
('path2','something else','Joe'), -- duplicate likely irrelevant but still updated
('path2','something else','Joe Someone'), -- already has space in composer skipped
('path100','an so on','x') -- no equivalent in master
;
INSERT INTO master VALUES
('path1','this','fred','bert'), -- ignored as no space
('path1','this','fred bloggs','Julia smith'),
('path2','something else','Alison','Joe Brown')
;
/* DEMO OF EXTRACTED ROWS */
WITH cte AS
(
SELECT
master.composer AS master_composer,
alib.rowid AS arowid
FROM alib
INNER JOIN
master ON hex(alib.__path) = hex(master.__path)
AND alib.l_title = lower(master.title)
AND alib.composer != master.composer
AND length(alib.composer) != length(master.composer)
AND master.composer LIKE '% %'
AND alib.composer NOT LIKE '% %'
)
SELECT * FROM cte;
WITH cte AS
(
SELECT
master.composer AS master_composer,
alib.rowid AS arowid
FROM alib
INNER JOIN
master ON hex(alib.__path) = hex(master.__path)
AND alib.l_title = lower(master.title)
AND alib.composer != master.composer
AND length(alib.composer) != length(master.composer)
AND master.composer LIKE '% %'
AND alib.composer NOT LIKE '% %'
)
UPDATE alib
SET composer =
(
SELECT cte.master_composer FROM cte WHERE cte.arowid = alib.rowid
)
WHERE alib.rowid IN (SELECT arowid FROM cte);
SELECT * FROM alib;
DROP TABLE IF EXISTS alib;
DROP TABLE IF EXISTS master;
Testing Results
The extracted data :-
alib table after UPDATES :-