Question

I have three tables, A B and C. For every entry in A x B (where x is a Cartesian product, or cross join) there is an entry in C.

In other words, the table for C might look like this, if there were 2 entries for A and 3 for B:

| A_ID | B_ID | C_Val |
----------------------|
|  1   |  1   |  100  |
|  1   |  2   |  56   |
|  1   |  3   |  19   |
|  2   |  1   |  67   |   
|  2   |  2   |  0    |
|  2   |  3   |  99   |

Thus, for any combination of A and B, there’s a value to be looked up in C. I hope this all makes sense.

In practice, the size of A x B may be relatively small for a database, but far too large to populate by hand for testing data. Thus, I would like to randomlly populate C‘s table for whatever data may already be in A and B.

My knowledge of SQL is fairly basic. What I’ve determined I can do so far is get that cartesian product as an inner query, like so:

(SELECT B.B_ID, C.C_ID
 FROM B CROSS JOIN C)

Then I want to say something like follows:

INSERT INTO A(B_ID, C_ID, A_Val) VALUES
    (SELECT B.B_ID, C.C_ID, FLOOR(RAND() * 100)
     FROM B CROSS JOIN C)

Not surprisingly, this doesn’t work. I don’t think its valid syntax to genereate a column on the fly like that, nor to try to insert a whole table as values.

How can I basically convert this normal programming pseudocode to proper SQL?

foreach(A_ID in A){
    foreach(B_ID in B){
        C.insert(A_ID, B_ID, Rand(100));
    }
}

Answer 1

The syntax problem is because:

INSERT INTO A(B_ID, C_ID, A_Val) VALUES
  (SELECT B.B_ID, C.C_ID, FLOOR(RAND() * 100)
   FROM B CROSS JOIN C)

Should be:

INSERT INTO A(B_ID, C_ID, A_Val) 
  SELECT B.B_ID, C.C_ID, FLOOR(RAND() * 100)
   FROM B CROSS JOIN C;

(You don’t use VALUES with INSERT/SELECT.)

However you will still have the problem that RAND() is not evaluated for every row; it will have the same value for every row. Assuming the combination of B_ID and C_ID is unique, you can use something like this:

INSERT INTO A(B_ID, C_ID, A_Val) 
  SELECT B.B_ID, C.C_ID, ABS(CHEKSUM(RAND(B.B_ID*C.C_ID))) % 100
   FROM B CROSS JOIN C;