sqlite / websql: separate queries produce different results than join queries - sql

Question

I have a very simple db that has two tables, one to store a user's auth (username and pw) and one to store preferences. Currently when my app initializes, it checks for settings first and then auth. But I want to grab it all at once. The way the app is designed, there is only ever 1 record in either table, so I don't use WHERE clauses in my queries.
The problem is, when I do queries separately, they give different results than if I join them.
Consider this scenario: a user logs in, sets some settings and then logs out. In may app this causes the auth table to be cleared but the settings remain intact.
Sign-out query (just clears auth table)
db.transaction(function(tx) {
tx.executeSql('DELETE FROM userdata');
}, errorDB);
However this outputs "0 rows found":
db.transaction(function(tx) {
tx.executeSql('SELECT USERDATA.*, PREFS.* FROM USERDATA, PREFS', [], function (tx, results) {
var len = results.rows.length;
console.log(len + " rows found.");
for (var i=0; i<len; i++){
console.log("Row = " + i + " ID = " + results.rows.item(i).id + " Data = " + results.rows.item(i));
}
}, errorDB);
}, errorDB);
Whereas this outputs a row:
db.transaction(function(tx) {
tx.executeSql('SELECT * FROM PREFS', [], function (tx, results) {
var len = results.rows.length;
console.log(len + " rows found.");
for (var i=0; i<len; i++){
console.log("Row = " + i + " ID = " + results.rows.item(i).id + " Data = " + results.rows.item(i));
}
}, errorDB);
}, errorDB);
Even if there is no auth shouldn't the join query find the prefs data? What's going on here?

Answer 1

When you list two tables A and B in the FROM clause, you get a cross join, which returns A×B records.
So if one table is empty, the result will be empty, too.
To ensure that you have a record even for an empty table, add another record with UNION ALL, then use LIMIT 1 return the first record.
For example,
SELECT Name, PW FROM UserData
UNION ALL
SELECT NULL, NULL
LIMIT 1
will return either a single record with the name/password, or a single record with two NULL values.
To join that with the other table, use a subquery:
SELECT *
FROM (SELECT Name, PW FROM UserData
UNION ALL
SELECT NULL, NULL
LIMIT 1),
Prefs