Introduction to Alternating Data Display in MySQL

When it comes to displaying data in a database table, one of the challenges that developers often face is how to alternate the display of certain columns or rows. In this article, we’ll explore a solution using MySQL, which involves enumerating the rows and then ordering by that enumeration.

Understanding the Problem

The problem at hand is to display the Site_car column in a table named car in an alternating manner, with each of the values 'onesite', 'twosite', and 'threesite' appearing in a specific order. The desired output should look like this:

The Approach

To solve this problem, we’ll need to use a combination of MySQL’s functions and variables. We’ll first enumerate the rows using a user-defined variable (@rn) and then order by that enumeration.

Step 1: Enumerating Rows

We start by creating a subquery that enumerates the rows based on the Site_car value. We use the @rn variable to keep track of the row number, which is initially set to 0. If the current Site_car value is different from the previous one, we increment the @rn variable by 1.

select c.*
from (select c.*,
         (@rn := if(@sc = site_car, @rn + 1,
                    if(@sc := site_car, 1, 1)
                   )
         ) as rn
      from (select c.*
            from car c
            order by site_car, id_car
           ) c cross join
           (select @sc := -1, @rn := 0) params
     ) c

In this subquery:

• We use the cross join to combine the table car with a parameterized query that sets the initial values of @sc and @rn.
• The if statement increments @rn only when the current value of site_car is different from the previous one.
• If the same value is encountered again, it sets @sc to the new value and resets @rn to 1.

Step 2: Ordering by Row Number

After enumerating the rows, we order them by the rn variable. We also use the field function to specify the desired ordering of the values in the Site_car column.

order by rn, field(site_car, 'onesite', 'twosite', 'threesite');

Here, we:

• Order by rn, which groups rows with the same site_car value together.
• Use the field function to specify that 'onesite' should appear first in each group, followed by 'twosite', and then 'threesite'.

Alternative Solution using MySQL 8+

For MySQL versions 8.0 and later, we can use the row_number() window function to achieve the same result.

select c.*
from car c
order by row_number() over (partition by site_car order by id_car),
         field(site_car, 'onesite', 'twosite', 'threesite');

In this solution:

• We use row_number() to assign a unique number to each row within each partition of the result set.
• We specify that rows with the same value in site_car should be ordered by id_car.

Conclusion

Displaying data in an alternating manner can be challenging, but MySQL provides several tools and functions to help achieve this goal. By using user-defined variables and window functions, we’ve demonstrated how to enumerate rows and order them based on a specific criteria.

Whether you’re working with older versions of MySQL or prefer the newer features, there’s a solution that suits your needs. With practice and experience, mastering these techniques will make it easier to work with data in various contexts.

Last modified on 2023-05-07