MySQL ROW_NUMBER() Window Function with CASE Expression
Use MySQL ROW_NUMBER() window function for multiple rows grouping to a single row using a CASE expression.
In this post, I am covering how to use the MySQL ROW_NUMBER() window function and an CASE expression to collapse multiple rows of data into a single row per group.
Data set up
In order for a more manageable set of data to work with, I am filtering the sakila database ‘city’ table for any rows whose country_id value is in the list of (26,27,28,29) using the IN() operator:
SELECT country_id, city
FROM city
WHERE country_id IN (26,27,28,29);
ROW_NUMBER() window function
In this Common Table Expression (CTE), the ROW_NUMBER() window function assigns an incrementing INTEGER value to each row in the result set. Since the PARTITION BY clause is present, sub-groups are formed based on the unique country_id column values and we are assigning order based on the ORDER BY city ASC clause:
WITH data_set AS (
SELECT country_id,
city,
ROW_NUMBER() OVER(PARTITION BY country_id ORDER BY city ASC) AS rn
FROM city
WHERE country_id IN (26,27,28,29)
)
SELECT * FROM data_set;Notice ROW_NUMBER() resets the incrementing INTEGER value for each distinct country_id sub-group.
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CASE expression and NULLs
Next, we use a CASE expression and retrieve each city column value based on what the rn alias value is. In those instances where the WHEN predicate is not true, NULL is returned for that city column value:
SELECT country_id,
CASE WHEN rn = 1 THEN city ELSE NULL END AS city_1,
CASE WHEN rn = 2 THEN city ELSE NULL END AS city_2,
CASE WHEN rn = 3 THEN city ELSE NULL END AS city_3,
CASE WHEN rn = 4 THEN city ELSE NULL END AS city_4,
CASE WHEN rn = 5 THEN city ELSE NULL END AS city_5,
CASE WHEN rn = 6 THEN city ELSE NULL END AS city_6
FROM data_set;
We are almost there with the query results we have so far. However, we need to further collapse the rows now into a single row per country_id value.
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Use Aggregate functions to collapse NULL rows
In order to collapse the multiple rows to a single row per group, we simply need to wrap each CASE expression in either the MAX() or MIN() aggregate function and GROUP BY the country_id value:
SELECT country_id,
MAX(CASE WHEN rn = 1 THEN city ELSE NULL END) AS city_1,
MAX(CASE WHEN rn = 2 THEN city ELSE NULL END) AS city_2,
MAX(CASE WHEN rn = 3 THEN city ELSE NULL END) AS city_3,
MAX(CASE WHEN rn = 4 THEN city ELSE NULL END) AS city_4,
MAX(CASE WHEN rn = 5 THEN city ELSE NULL END) AS city_5,
MAX(CASE WHEN rn = 6 THEN city ELSE NULL END) AS city_6
FROM data_set
GROUP BY country_id;
As shown in this similar query, replacing the MAX() aggregate function with MIN() yields the same results:
SELECT country_id,
MIN(CASE WHEN rn = 1 THEN city ELSE NULL END) AS city_1,
MIN(CASE WHEN rn = 2 THEN city ELSE NULL END) AS city_2,
MIN(CASE WHEN rn = 3 THEN city ELSE NULL END) AS city_3,
MIN(CASE WHEN rn = 4 THEN city ELSE NULL END) AS city_4,
MIN(CASE WHEN rn = 5 THEN city ELSE NULL END) AS city_5,
MIN(CASE WHEN rn = 6 THEN city ELSE NULL END) AS city_6
FROM data_set
GROUP BY country_id;
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