How do I make a row generator in MySQL?

Question

Is there a way to generate an arbitrary number of rows that can be used in a JOIN similar to the Oracle syntax:

SELECT LEVEL FROM DUAL CONNECT BY LEVEL<=10

Answer 1

Hate to say this, but MySQL is the only RDBMS of the big four that doesn't have this feature.

In Oracle:

SELECT  *
FROM    dual
CONNECT BY
        level < n

In MS SQL (up to 100 rows):

WITH hier(row) AS
        (
        SELECT  1
        UNION ALL
        SELECT  row + 1
        FROM    hier
        WHERE   row < n
        )
SELECT  *
FROM    hier

or using hint up to 32768

WITH hier(row) AS
        (
        SELECT  1
        UNION ALL
        SELECT  row + 1
        FROM    hier
        WHERE   row < 32768
        )
SELECT  *
FROM    hier
OPTION (MAXRECURSION 32767) -- 32767 is the maximum value of the hint

In PostgreSQL:

SELECT  *
FROM    generate_series (1, n)

In MySQL, nothing.

Answer 2

In MySql, it is my understand that you can get more than one row with a SELECT with no table (or DUAL).

Therefore, to get multiple rows, you do need a real or temporary table with at least the required number of rows.

However, you do not need to build a temporary table as you can use ANY existing table which has at least the number of rows required. So, if you have a table with at least the required number of rows, use:

SELECT  @curRow := @curRow + 1 AS row_number
FROM    sometable 
JOIN    (SELECT @curRow := 0) r
WHERE   @curRow<100;

Just replace "sometable" with the name of any table of yours with at least the required number of rows.

PS: The "r" is a table "alias": I could have used "AS r". Any subquery in a FROM or JOIN clause creates a "derived table" which, as with all tables, must have a name or alias. (See MySql manual: 13.2.9.8. Subqueries in the FROM Clause)

Answer 3

Since this is currently one of the first results in Google for "mysql row generator", I'll add an update.

If your flavor of MySQL happens to be MariaDB, they have this feature. It's called the "Sequence Storage engine" and it's used like this:

select * from seq_1_to_10;

With the results:

+-----+
| seq |
+-----+
|   1 |
|   2 |
|   3 |
|   4 |
|   5 |
|   6 |
|   7 |
|   8 |
|   9 |
|  10 |
+-----+
10 rows in set (0.00 sec)

Until version 10.0 it was a separate plugin that needed to be explicitly installed, but from 10.0 onwards it's built in. Enjoy!

Answer 4

I had a table with a column (c5) that contained a number x, I needed a SQL expression that repeated the same row x numbers of times.

My table A contained:

c1  c2  c3  c4  c5
16  1   2   16  3
16  1   2   17  2 
16  1   2   18  1

And I needed:

c1  c2  c3  c4  c5  n
16  1   2   16  3   1
16  1   2   16  3   2
16  1   2   16  3   3
16  1   2   17  2   1
16  1   2   17  2   2
16  1   2   18  1   1

I solved that with the expression:

SELECT
    c1, c2, c3, c4, c5, row_number AS n
FROM
    (
        SELECT
            @curRow := @curRow + 1 AS row_number
        FROM
            tablea
        JOIN (SELECT @curRow := 0) r
        WHERE
            @curRow < (
                SELECT
                    max(field1)
                FROM
                    tablea
            )
    ) AS vwtable2
LEFT JOIN tablea d ON vwtable2.row_number <= tablea.field1;

Answer 5

If I'm understanding you, you want a list of consequtive numbers?

Just make the list:

create table artificial_range (id int not null primary key auto_increment, idn int);
insert into artificial_range (idn) values (0); --first row
insert into artificial_range(idn) select idn from artificial_range; --2nd
insert into artificial_range(idn) select idn from artificial_range; -- now 4 rows
insert into artificial_range(idn) select idn from artificial_range; --8
insert into artificial_range(idn) select idn from artificial_range; --16
insert into artificial_range(idn) select idn from artificial_range; --32
insert into artificial_range(idn) select idn from artificial_range; --64
insert into artificial_range(idn) select idn from artificial_range; --128

... etc, until you have, say, 1024.

update artificial_range set idn = id - 1 ; 

-- now you have a series staring at 1 (id) and a series starting at 0

Now join to it, or join to transformations of it:

    create view days_this_century as 
select date_add('2000-01-01', interval a.idn day) as cdate 
from artificial_range;

Answer 6

I don't know if this helps but you can number the rows from each select statement with sth. like:

SET @NUM = 0;

SELECT @NUM:=@NUM+1 rowNumber, * FROM ...

And later join them on this one. At large databases this can be very slow.

Answer 7

To generate 10 rows:

SELECT a AS a UNION ALL SELECT 1 UNION ALL SELECT 2 UNION ALL SELECT 3 UNION ALL SELECT 4 UNION ALL SELECT 5 UNION ALL SELECT 6 UNION ALL SELECT 7 UNION ALL SELECT 8 UNION ALL SELECT 9 from dual

You can generate 100 rows making a join with another 10 rows:

select t2.a*10 + t1.a from (SELECT 0 AS a UNION ALL SELECT 1 UNION ALL SELECT 2 UNION ALL SELECT 3 UNION ALL SELECT 4 UNION ALL SELECT 5 UNION ALL SELECT 6 UNION ALL SELECT 7 UNION ALL SELECT 8 UNION ALL SELECT 9) t1, (SELECT 0 AS a UNION ALL SELECT 1 UNION ALL SELECT 2 UNION ALL SELECT 3 UNION ALL SELECT 4 UNION ALL SELECT 5 UNION ALL SELECT 6 UNION ALL SELECT 7 UNION ALL SELECT 8 UNION ALL SELECT 9) t2

And then 1000 rows with another join, ...