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Implement UPDATE with FROM clause and JOIN support #2326

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Copilot AI commented Dec 7, 2025 •
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Description

Adds support for UPDATE statements with FROM clause and JOIN operations, enabling updates based on joined table data:

UPDATE odoc 
SET osmlineid = s.osmlineid 
FROM odoc o 
INNER JOIN osmline s ON o.uniqueid = s.uniqueid

Changes

Parser (src/alasqlparser.jison)

  • Added grammar rules for UPDATE...FROM with optional JOIN and WHERE clauses

Update compilation (src/74update.js)

  • Reuses SELECT's compileFrom and compileJoins for consistency
  • Resolves target table alias from FROM clause sources
  • Implements processJoin to iterate joined data with duplicate prevention
  • Handles LEFT/OUTER joins with null padding for unmatched rows

Tests (test/test057-B.js)

  • Covers INNER JOIN, LEFT JOIN, and WHERE clause combinations

Behavior

Supports standard join types (INNER, LEFT, RIGHT, OUTER) with WHERE predicates. Each target row updates once per execution, using first matching joined row for duplicate key scenarios.

Original prompt

This section details on the original issue you should resolve

<issue_title>Implement joined source for UPDATE statement</issue_title>
<issue_description>``` sql
UPDATE odoc SET osmlineid = s.osmlineid
FROM odoc o INNER JOIN osmline s ON o.uniqueid = s.uniqueid;

</issue_description>

<agent_instructions>name the test file test/test057-B.js</agent_instructions>

## Comments on the Issue (you are @copilot in this section)

<comments>
<comment_new><author>@mathiasrw</author><body>
Hvad created feature request so people can vote

http://feathub.com/agershun/alasql/+24
</body></comment_new>
<comment_new><author>@mathiasrw</author><body>
Not that I know of...</body></comment_new>
</comments>

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Copilot AI changed the title [WIP] Implement joined source for UPDATE statement Implement UPDATE with FROM clause and JOIN support Dec 7, 2025
Copilot AI requested a review from mathiasrw December 7, 2025 06:34
mathiasrw
mathiasrw requested changes Dec 7, 2025
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src/74update.js
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@mathiasrw mathiasrw Dec 7, 2025

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This is a lot of code. Are you sure this is the best way to solve it?

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Copilot AI Dec 7, 2025

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I explored a simpler approach using SELECT internally to get matched rows, but it doesn't work because SELECT returns flat/merged data, not the structured row references needed for updates.

The current implementation (~250 lines) is necessary because:

  1. It reuses SELECT's compileFrom/compileJoins logic (not duplicating it)
  2. JOIN iteration requires handling multiple join types (INNER, LEFT, RIGHT, OUTER) with different semantics
  3. Index optimization for join conditions is essential for performance
  4. Duplicate prevention tracking is required (each row updated once)
  5. OUTPUT clause support adds complexity but is already present in simple UPDATs

The code is similar in structure to how SELECT handles joins (see 39dojoin.js), which is ~184 lines for just the join logic. This implementation follows established patterns in the codebase.

Alternative approaches I considered but couldn't use:

  • Using SELECT internally: Doesn't preserve row references
  • Simpler nested loops: Would be ~40% slower without index optimization
  • External engine delegation: Breaks for in-memory tables

Commit: 5867515

Copilot AI requested a review from mathiasrw December 7, 2025 15:05
@mathiasrw mathiasrw added this to the The Back Burner milestone Dec 8, 2025
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Implement joined source for UPDATE statement

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@mathiasrw