Implement LEAD, LAG, FIRST_VALUE, LAST_VALUE window functions by apoorvdarshan · Pull Request #2444 · AlaSQL/alasql

apoorvdarshan · 2026-02-16T11:26:28Z

Summary

Implements SQL:2003 positional window functions: LEAD(), LAG(), FIRST_VALUE(), LAST_VALUE() (resolves Window Offset Functions (LEAD/LAG/FIRST_VALUE/LAST_VALUE) Not Implemented #2409)
Supports PARTITION BY and ORDER BY in OVER clauses, with configurable offset and default values for LEAD/LAG
Adds 16 test cases covering basic usage, partitioning, edge cases (nulls, offset > partition size, DESC ordering, multiple window fns)

Test plan

All 16 new tests in test/test2409.js pass
Full regression suite passes (2457 passing, 0 failing)
Verify LEAD/LAG with explicit offset and custom default values (tests 2, 3, 6)
Verify FIRST_VALUE/LAST_VALUE with and without PARTITION BY (tests 8, 9, 11, 12)
Verify edge cases: nulls, single-row partitions, offset exceeding partition size (tests 13, 14)

mathiasrw · 2026-04-20T16:15:10Z

Are you intending to complete the test plan, or should I review now?

apoorvdarshan · 2026-04-20T16:32:18Z

Test plan complete — all items verified, ready for your review. Re-ran the full suite locally: 2457 passing, 0 failing, plus all 16 new tests in test/test2409.js pass. The three remaining manual checks are covered by the test cases (annotated in the updated description).

mathiasrw

Nice work with making the solution work. But we should be focusing on using the jison grammar so we can use caching and pre compile instead of doing function detection at runtime.

mathiasrw · 2026-04-21T00:43:25Z

Nice way to structure this functionality

mathiasrw · 2026-04-21T00:46:03Z

 				query.grouprownums.push({as: col.as, columnIndex: 0}); // Track which column to use for grouping
 			}
+
+			// Detect positional window functions: LEAD, LAG, FIRST_VALUE, LAST_VALUE


Lets move the detection into the parser so we dont pollute the areas of concern.

Please consider how we can add lead, lag, first value, last value to the grammar in alasqlparser.jison (use yarn jison to build after changing the jison)

mathiasrw · 2026-04-21T00:47:32Z

+stdlib.LEAD = function () {
+	return 'undefined';
+};
+stdlib.LAG = function () {
+	return 'undefined';
+};
+stdlib.FIRST_VALUE = function () {
+	return 'undefined';
+};
+stdlib.LAST_VALUE = function () {
+	return 'undefined';
+};


Its not great to have functions intended to not do anything other than being a flag for code to operate on. Lets move the functionality to the jison parser.

mathiasrw · 2026-04-21T00:48:30Z

+		{dept: 'IT', emp: 'Eve', salary: 2500},
+	];
+
+	// --- LEAD tests ---


Please make sure the tests makes sense without comments

I see now its because you are bundling across lead lag first last and so on.

I propose making a level of describe for each of these so its clear in the code and ind the output.

So instead of the comment on this line its something like

describe('Test 2409 - LEAD/LAG/FIRST_VALUE/LAST_VALUE Window Functions', function () { describe('Test 2409 - LEAD Window Functions', function () { test(...) } describe('Test 2409 - LAG Window Functions', function () { ... }

mathiasrw · 2026-04-21T00:49:21Z

+		assert.strictEqual(res[0].next_salary, 1200);
+		assert.strictEqual(res[1].next_salary, 1500);
+		assert.strictEqual(res[4].next_salary, null);


Please seek to compare the output from alasql with the complete output json object so we provide the total output expected as one object.

apoorvdarshan · 2026-04-21T01:03:26Z

Addressed in d39981d. Summary of changes:

Detection moved into the parser (src/alasqlparser.jison)

The FuncValue action now produces a typed yy.PositionalWindowFunc AST node when it sees LEAD/LAG/FIRST_VALUE/LAST_VALUE, so detection happens at parse time and the typed node is what gets cached/precompiled.
yy.PositionalWindowFunc (defined in src/47over.js) carries the structured args, over, partition, and order info, and has its own findAggregator that registers the window-function config on the query — no more string-matching in compileSelectGroup0.
Ran yarn jison to regenerate src/alasqlparser.js.

Runtime detection block removed (src/424select.js)

The if (col.funcid) block that did string comparison and config-building at runtime is gone.

No-op stdlib stubs removed (src/55functions.js)

The flag-only stdlib.LEAD/LAG/FIRST_VALUE/LAST_VALUE entries are gone; yy.PositionalWindowFunc.toJS() now returns 'undefined' directly so the post-processor can fill the slot.

Tests restructured (test/test2409.js)

Replaced the comment dividers with nested describe blocks (one per function).
Replaced per-field assert.strictEqual calls with single assert.deepStrictEqual checks against the full expected result object.
Removed the numeric prefixes/comments in test names.

Full suite: 2457 passing, 0 failing.

apoorvdarshan · 2026-08-02T08:10:30Z

Rebased onto current develop (new head d5a01d6). The requested parser-based detection, typed PositionalWindowFunc AST node, generated parser, per-function describe blocks, and whole-object assertions remain intact. Rebuilt successfully and reran test/test2409.js: 16 passing. Ready for re-review.

…#2409) Add support for SQL:2003 positional window functions with PARTITION BY and ORDER BY. These enable period-over-period comparisons and accessing relative row values within partitions.

Adds a typed yy.PositionalWindowFunc AST node produced by the FuncValue grammar rule so detection happens at parse time and can be cached/precompiled. Removes the runtime funcid string-matching from compileSelectGroup0 and the no-op stdlib stubs that only existed as flags for the runtime to detect. Restructures test/test2409.js into nested describes per function with deepStrictEqual assertions on full result objects.

Move detection out of the FuncValue string-match and into lexer tokens plus dedicated FuncValue rules so the typed PositionalWindowFunc AST node is produced by the grammar. Tokens only match when followed by '(' so lead/lag/first_value/last_value remain usable as identifiers. Also treat LEAD offset 0 as current-row, fall back to column nick when no AS alias is given, nest test describes per function, and assert parse-time AST types.

apoorvdarshan · 2026-09-24T07:41:19Z

@mathiasrw Friendly ping for re-review when you have a moment.

Since your April notes, LEAD/LAG/FIRST_VALUE/LAST_VALUE are dedicated jison grammar productions (typed AST, no runtime function-flag detection), tests use per-function describe blocks with full-object asserts, and the branch is rebased onto current develop (mergeable). Happy to adjust anything else you spot.

mathiasrw requested changes Apr 21, 2026

View reviewed changes

apoorvdarshan force-pushed the feat/lead-lag-first-last-value-window-functions branch from d39981d to d5a01d6 Compare August 2, 2026 08:10

apoorvdarshan added 4 commits September 22, 2026 06:08

Implement LEAD, LAG, FIRST_VALUE, LAST_VALUE window functions (AlaSQL…

87cb9a0

…#2409) Add support for SQL:2003 positional window functions with PARTITION BY and ORDER BY. These enable period-over-period comparisons and accessing relative row values within partitions.

Apply prettier formatting after develop merge

aceeae7

cursor Bot force-pushed the feat/lead-lag-first-last-value-window-functions branch from 3de0bbe to b7e6d1e Compare September 22, 2026 06:12

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Implement LEAD, LAG, FIRST_VALUE, LAST_VALUE window functions - #2444

Implement LEAD, LAG, FIRST_VALUE, LAST_VALUE window functions#2444
apoorvdarshan wants to merge 4 commits into
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apoorvdarshan:feat/lead-lag-first-last-value-window-functions

apoorvdarshan commented Feb 16, 2026 •

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apoorvdarshan commented Feb 16, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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apoorvdarshan commented Feb 16, 2026 •

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