Group by Case Statement MySQL - mysql

Question

I am trying to group by the CASE statement but not having much luck. I have an orders table that I am trying to group the orders by total value for the month and categorise them based on their value.
SELECT
CASE
WHEN sum(order_total_price) IS NULL
THEN 'Unknown'
WHEN sum(order_total_price) <= 1000
THEN 'Not more than 1,000'
WHEN sum(order_total_price) <= 2000
THEN 'Between 1,001 and 2000'
WHEN sum(order_total_price) <= 3000
THEN 'Between 2001 and 3000'
WHEN sum(order_total_price) <= 4000
THEN 'Between 3001 and 4000'
WHEN sum(order_total_price) <= 5000
THEN 'Between 4001 and 5000'
ELSE 'Over 5000'
END
AS total_sales,
COUNT(*) as total
FROM orders
WHERE YEAR(order_time)=2014 and MONTH(order_time)=07
GROUP BY total_sales

Answer 1

The shorthand for this is
....
GROUP BY 1
Using the output column position. Otherwise you'd have to repeat the entire case block in the GROUP BY clause.

Answer 2

Your query should be returning a single row. Luck doesn't have anything do with it. What matters is the specification. And apart from the SQL query, it's not clear what resultset you want returned.
I'm guessing (and this is just a guess) that you want to return multiple rows, one for each of the order size categories derived by the CASE expression.
Perhaps the resultset returned by a query something like this:
SELECT CASE
WHEN o.order_total_price IS NULL
THEN 'Unknown'
WHEN o.order_total_price <= 1000
THEN 'Not more than 1,000'
WHEN o.order_total_price <= 2000
THEN 'Between 1,001 and 2000'
WHEN o.order_total_price <= 3000
THEN 'Between 2001 and 3000'
WHEN o.order_total_price <= 4000
THEN 'Between 3001 and 4000'
WHEN o.order_total_price <= 5000
THEN 'Between 4001 and 5000'
ELSE 'Over 5000'
END AS order_value_category
, SUM(o.order_total_price) AS total_sales
, COUNT(*) AS count_sales
FROM orders o
WHERE o.order_time >= '2014-07-01'
AND o.order_time < '2014-07-01' + INTERVAL 1 MONTH
GROUP BY order_value_category
For performance, to enable MySQL to make use of a suitable index to satisfy the predicate on the order_time category, we'd need to reference the "bare" order_time column in a range scan predicate.
Note that the GROUP BY clause implies an ORDER BY clause; if you want the rows returned in order other than by the derived order_value_category column, you'd need to specify a suitable ORDER BY clause.

Answer 3

If you use GROUP BY, output rows are sorted according to the GROUP BY columns as if you had an ORDER BY for the same columns. To avoid the overhead of sorting that GROUP BY produces, add ORDER BY NULL:
SELECT a, COUNT(b) FROM test_table GROUP BY a ORDER BY NULL;
Relying on implicit GROUP BY sorting in MySQL 5.6 is deprecated. To achieve a specific sort order of grouped results, it is preferable to use an explicit ORDER BY clause. GROUP BY sorting is a MySQL extension that may change in a future release; for example, to make it possible for the optimizer to order groupings in whatever manner it deems most efficient and to avoid the sorting overhead.
http://academy.comingweek.com/sql-groupby-clause/

Answer 4

As Jim Garrison said,
GROUP BY 1
would be best.
As an alternative, below will also work if you are not comfortable with numbers in group by. Put your original query without GROUP BY as inner query. Use the count and group by in an outer query.
SELECT A.TOTAL_SALES, COUNT(1) FROM
(SELECT
CASE
WHEN sum(order_total_price) IS NULL
THEN 'Unknown'
WHEN sum(order_total_price) <= 1000
THEN 'Not more than 1,000'
WHEN sum(order_total_price) <= 2000
THEN 'Between 1,001 and 2000'
WHEN sum(order_total_price) <= 3000
THEN 'Between 2001 and 3000'
WHEN sum(order_total_price) <= 4000
THEN 'Between 3001 and 4000'
WHEN sum(order_total_price) <= 5000
THEN 'Between 4001 and 5000'
ELSE 'Over 5000'
END
AS total_sales
FROM orders
WHERE YEAR(order_time)=2014 and MONTH(order_time)=07
) AS A
GROUP BY A.total_sales