In a join, an unqualified column that is NULL in the first source takes the value of the second source #2873

In a join, an unqualified column that is NULL in the first source takes its value from the second source. WHERE x IS NULL stops finding the row, and x + 0 reads the other table's value.

var A = [{id: 1, x: null}, {id: 2, x: 20}];
var B = [{id: 1, x: 100}, {id: 2, x: 200}];
alasql('SELECT t0.id FROM ? t0 JOIN ? t1 ON t0.id = t1.id WHERE x IS NULL', [A, B]);
// [] (4.20.0: [{id: 1}])
alasql('SELECT t0.id, x + 0 AS v FROM ? t0 JOIN ? t1 ON t0.id = t1.id', [A, B]);
// [{id: 1, v: 100}, {id: 2, v: 20}] (4.20.0: v is missing for id 1)

Expected: the first query gives [{id: 1}], and v is NULL for id 1.

Since #2579 (in 4.21.0). An unqualified column that is not in defcols compiles to p['t0']?.['x'] ?? p['t1']?.['x'] (Column.toJS and Column.fromAnySource in src/expression/expression.js, and its use in src/select/columns.js), so a NULL falls through to the next source. It affects WHERE, ON, ORDER BY, GROUP BY and computed columns. The fix: go to the next source only when the row has no such property ('x' in p['t0']), not when the value is NULL, and add a test with NULL in the first source (test E of #2579 has no NULL).

Breaking?

No. It restores the result of 4.20.0.