fix: concat_ws should skip null values and honour a null separator

src/functions/string.ts

@@ -1,4 +1,4 @@
-import { DataType, FunctionDefinition } from '../interfaces-private';
+import { DataType, FunctionDefinition, nil } from '../interfaces-private';
 
 export const stringFunctions: FunctionDefinition[] = [
     {
@@ -27,6 +27,11 @@ export const stringFunctions: FunctionDefinition[] = [
         argsVariadic: DataType.text,
         returns: DataType.text,
         allowNullArguments: true,
-        implementation: (separator: string, ...x: string[]) => x?.join(separator),
+        implementation: (separator: string | nil, ...x: (string | nil)[]) => {
+            if (separator === null || separator === undefined) {
+                return null;
+            }
+            return x.filter(v => v !== null && v !== undefined).join(separator);
+        },
     },
 ]

src/tests/function-calls.spec.ts

@@ -35,6 +35,21 @@ describe('Functions', () => {
             .toEqual([{ concat: 'text-123-end' }]);
     });
 
+    it('concat_ws skips null arguments', () => {
+        expect(many(`select concat_ws(',', 'a', null, 'b') as v;`))
+            .toEqual([{ v: 'a,b' }]);
+    });
+
+    it('concat_ws returns null when the separator is null', () => {
+        expect(many(`select concat_ws(null, 'a', 'b') as v;`))
+            .toEqual([{ v: null }]);
+    });
+
+    it('concat_ws returns an empty string when every value is null', () => {
+        expect(many(`select concat_ws(',', null, null) as v;`))
+            .toEqual([{ v: '' }]);
+    });
+
     it('GREATEST 2 arguments', () => {
         expect(many(`select GREATEST(0, -1);`))
             .toEqual([{ greatest: 0 }]);