Create a view with column num_rows - MySQL

Question

I need to create a view that has a column named row_num where it will be inserted the row number, just like an auto increment in a normal table.

Let\'s say I\'ve thi

Answer 1

When using the solution from @dazito you might encounter an issue with the counter continually incrementing from query to query, for example when your application reuses a session, like with JPA / Hibernate. For example:

Query 1:

| country | name | price | row_num |
------------------------------------
| US      | john |  20   |    1    |
| France  | Anne |  10   |    2    |
| Sweden  | Alex |  5    |    3    |

Query 2:

| country | name | price | row_num |
------------------------------------
| US      | john |  20   |    4    |
| France  | Anne |  10   |    5    |
| Sweden  | Alex |  5    |    6    |

etc.

One solution to this is to join the main query with a (one-time) call to the counter function and parameterize the function (the 'reset' parameter below) to let it know it is the first call.

delimiter //
CREATE FUNCTION `func_inc_var_session`(reset BIT) RETURNS int
    NO SQL
    NOT DETERMINISTIC
     begin
      IF reset THEN
        SET @var := 0;
      ELSE
        SET @var := IFNULL(@var,0) + 1;
      END IF;
      return @var;
     end
     //
delimiter ;

Now you can call the function in your view query with the reset parameter set to 1 to set the function's counter variable back to 0, and with 0 to increment the counter. The function will only get called once with 1 as a parameter when joining with it as below:

CREATE OR REPLACE VIEW country_view (country, name, price, row_num) 
AS SELECT country, name, price, func_inc_var_session(0) 
FROM country
JOIN (SELECT func_inc_var_session(1)) r

Now you are guaranteed row number 1, 2, 3 every time.