MySQL Join tables without having common column at all, just by order

Question

I have the same problem described in this other question:

MySQL / PHP Joining tables without always having common column

But my tables don't have ane field in common, and i only want to join them "the first one with the firs one", "the second one with the second one" and so on.

Example:

Table_1

• john
• mary
• charles

Table_2

• smith
• goodman
• bush

TableResult

• john | smith
• mary | goodman
• charles | bush

Tables should allways have the same ammount of items, but if not, the unexisting items could be filled with null, as "henrry | null".

Perhaps it could be done adding first a field to number each row, or something so, but performance is critical here.

Answer 1

This really isn't a good idea, and can never be guaranteed consistent results, but you can get the row number of each table and use that as your joining field.

Try somemthing like this:

SELECT Field1, Field2
FROM (SELECT Field1, @curRow := @curRow + 1 AS row_number
      FROM Table1
         JOIN    (SELECT @curRow := 0) r) t1
 JOIN (SELECT Field2, @curRow2 := @curRow2 + 1 AS row_number
      FROM Table2
         JOIN    (SELECT @curRow2 := 0) r) t2 on t1.row_number = t2.row_number

And here is the SQL Fiddle .

This will only work if both tables have the same number of rows. If not, you'll be missing data...

Good luck.

Answer 2

If you want performance just use integer primary keys but you do need keys. You CAN NOT AND SHOULD NOT do it the way you described. :)

Answer 3

Edit:

Given that your requirements are specifically to achieve random results, I think you can make use of a cross join .

The following fiddle captures my effort to demonstrate this.

--

Otherwise, if your data is truly constant (in which case, why are you using a database?) then it could produce consistent results - but you could just as easily define keys and be done with it. If not, consider what happens with your expectation that "the unexisting [sic] items could be filled with null" when you grow the database:

Before

Table 1

foo
bar

Table 2

qux

Result

foo qux
bar null

After

Table 1

foo
bar
baz

Table 2

qux
quxx

Result

foo qux
bar quxx
baz null

You can't add baz quxx to the database.