Question

I’m no SQL expert, but have a bug in an iPhone app where and UPDATE statement has no effect on the db.
I have been using the SQLlite manger plugin for FireFox to try dbug by repeatedly amending and running the UPDATE on the db. I also ran the statement thorough and SQL Validator which said it complied to the core SQL standard.

Can you spot anything wrong with the statement given below?

UPDATE sections
SET
    title = 'What is acne ? ABC',
    text = 'Pus on your face',
    created = '2010-03-10 18:46:55',
    modified = '2011-07-04 17:38:44',
    position = 1,
    condition_id = 4
WHERE id = 10;

Answer 1

There is some confusion and inconsistency in the way SQLite and the various implementations by Mozilla, Google, Adobe, and others handle numeric primary keys in databases whose tables were created outside of these implementations and where the primary keys were defined as an integer type but not as “INTEGER” [verbatim] — that is, they were defined as INT or INT16 or INT32 etc.

        INTEGER PRIMARY KEY in mothership SQLite is an alias for the rowid.
        INT PRIMARY KEY in mothership SQLite is not an alias for the rowid.

A consortium member (or any implementor) may or may not follow this rule. (SQLite is in the public domain, of course.)

See section 2.0 here: http://www.sqlite.org/datatypes.html

and see section on RowId and Primary Key here: http://www.sqlite.org/lang_createtable.html#rowid

A PRIMARY KEY column only becomes an integer primary key if the
declared type name is exactly “INTEGER”. Other integer type names like
“INT” or “BIGINT” or “SHORT INTEGER” or “UNSIGNED INTEGER” causes the
primary key column to behave as an ordinary table column with integer
affinity and a unique index, not as an alias for the rowid. [emphasis
added]

An implementor who does not follow the rule might not have even been aware that they were breaking the rule in the first place, since it is a “gotcha” arcane sort of rule. Anyway, what this means practically is that one implementation may treat a supplied value as an alias for the rowid and another implementation might not. If given the value 10, one might retrieve the tuple whose rowid = 10 and one might retrieve the tuple where the specified column’s value = 10. This, of course, leads to spurious results in queries — and they might look like perfectly good and plausible results but they are dead wrong.

Consider the following simple test: using flagship SQLite’s utilities, not those provided by one of the implementors, execute the following DDL and DML statements; then, in your implementation, open the database and execute the DML statements again to compare the DML results:

        CREATE TABLE TEST
        ("id" INT PRIMARY KEY, "name" text)  -- ** NOTE "INT" not "INTEGER"

        INSERT INTO TEST
        (id, name)
        VALUES
        (7,'seven')


         ** *** N.B. THE ROWID OF THE ROW INSERTED ABOVE =  1  *** **


        select rowid, id, name from test

       result:  1 | 7 | seven


        select * from TEST 

        result:  7 | seven


        select * from TEST where id = 7

        result:  ?????  [ymmv]



        select * from TEST where id = 1


        result: ?????  [ymmv]

Depending on how the specific implementation treats an INT primary key the third select statement above (select * from TEST where id = 7) may return one row or it may return nothing!

If the implementation treats the INT PK as an alias for the row id, well, there is no row whose rowid = 7, and so it will return nothing. If the implementation treats the INT PK as a normal value, it will find the row.

Now, if you were to insert more rows into table TEST, you would eventually create a row whose rowid = 7. In one of these wayward implementations, when you use this where-clause — where id = 7 — you might think you were addressing the tuple whose id=7, but you’d actually be addressing the tuple whose rowid=7. You would get the wrong tuple and you might not realize it. Consider the possibilities when joining a child table to a parent table: the child table contains foreign key value of 7. What tuple does an inner join return from the parent table? It depends on whether the implementation honors the distinction between INT and INTEGER primary keys.

Last year, I documented this thoroughly for Adobe AIR, BTW, and also reported it on the SQLite news group. It is possible that some implementations have changed the behavior in the interim.

When creating SQLite tables, it is best to use INTEGER [verbatim] for primary keys, not any of the other recognized int types.