Fix GROUP_CONCAT to handle NULL values per SQL semantics

src/55functions.js

@@ -283,8 +283,20 @@ stdfn.CONCAT_WS = function () {
 // Aggregator for joining strings
 alasql.aggr.group_concat = alasql.aggr.GROUP_CONCAT = function (v, s, stage) {
 	if (stage === 1) {
+		// Initialize: skip null values
+		if (v === null || v === undefined) {
+			return null;
+		}
 		return '' + v;
 	} else if (stage === 2) {
+		// Accumulate: skip null values
+		if (v === null || v === undefined) {
+			return s;
+		}
+		// If accumulator is null/undefined, start with current value
+		if (s === null || s === undefined) {
+			return '' + v;
+		}
 		s += ',' + v;
 		return s;
 	}

test/test942.js

@@ -0,0 +1,61 @@
+if (typeof exports === 'object') {
+	var assert = require('assert');
+	var alasql = require('..');
+}
+
+describe('Test 942 - GROUP_CONCAT null handling', function () {
+	it('A) GROUP_CONCAT should skip null values', function () {
+		var data = [
+			{a: 1, b: 'x'},
+			{a: 1, b: null},
+			{a: 1, b: 'z'},
+		];
+		var res = alasql('SELECT a, GROUP_CONCAT(b) AS b FROM ? GROUP BY a', [data]);
+		assert.equal(res[0].b, 'x,z');
+	});
+
+	it('B) GROUP_CONCAT with all nulls should return null', function () {
+		var data = [
+			{a: 1, b: null},
+			{a: 1, b: null},
+		];
+		var res = alasql('SELECT a, GROUP_CONCAT(b) AS b FROM ? GROUP BY a', [data]);
+		assert.strictEqual(res[0].b, null);
+	});
+
+	it('C) GROUP_CONCAT DISTINCT with nulls should skip nulls', function () {
+		var data = [
+			{a: 1, b: 'x'},
+			{a: 1, b: null},
+			{a: 1, b: 'x'},
+			{a: 1, b: null},
+		];
+		var res = alasql('SELECT a, GROUP_CONCAT(DISTINCT b) AS b FROM ? GROUP BY a', [data]);
+		assert.equal(res[0].b, 'x');
+	});
+
+	it('D) GROUP_CONCAT with mixed values and nulls', function () {
+		var data = [
+			{a: 1, b: 10},
+			{a: 1, b: null},
+			{a: 1, b: 20},
+			{a: 2, b: null},
+			{a: 2, b: 30},
+		];
+		var res = alasql('SELECT a, GROUP_CONCAT(b) AS b FROM ? GROUP BY a ORDER BY a', [data]);
+		assert.equal(res[0].b, '10,20');
+		assert.equal(res[1].b, '30');
+	});
+
+	it('E) GROUP_CONCAT without GROUP BY with nulls', function () {
+		var data = [{b: 'a'}, {b: null}, {b: 'b'}, {b: null}, {b: 'c'}];
+		var res = alasql('SELECT GROUP_CONCAT(b) AS b FROM ?', [data]);
+		assert.equal(res[0].b, 'a,b,c');
+	});
+
+	it('F) GROUP_CONCAT with only null values (no GROUP BY)', function () {
+		var data = [{b: null}, {b: null}, {b: null}];
+		var res = alasql('SELECT GROUP_CONCAT(b) AS b FROM ?', [data]);
+		assert.strictEqual(res[0].b, null);
+	});
+});