Aggregate function to count occurrences of specific word

aggregategroup by

I have a table upon which I'm performing a GROUP BY statement. One of the columns in my table can contain one of 5 different words (essentially an enumerated type). Is there a way I can use an aggregate to get the occurrences of each enum, but display each count in a new column?

For example, given the table:

+---------+
|id | enum|
+---------+
| 1 | A   |
| 1 | B   |
| 1 | C   |
| 1 | A   |
+---------+

I'd like to get something like this after grouping by id:

+------------+---------+---------+
|id | a_count| b_count | c_count |
+------------+---------+---------+
| 1 |   2    |    1    |    1    |
+--------------------------------+

Best Answer

Sure, have 3 SUMs of the needed values converted to 1, and others to 0.

select
  id,
  sum(case when enum = 'A' then 1 else 0 end) as a_count,
  sum(case when enum = 'B' then 1 else 0 end) as b_count,
  sum(case when enum = 'C' then 1 else 0 end) as c_count
from
  table
group by
  id
;

Related Solutions

Mysql – Is it possible to reduce a group to a row based on some criteria

The traditional solution, the one you may find in books, is to do a self join: first find that "max date per group" of yours, then join to self table on rows with said max date.

However, some hacks allow you to avoid that. Consider the following query:

SELECT
  MAX(date_added) AS date_added,
  SUBSTRING_INDEX(GROUP_CONCAT(some_column ORDER BY date_added DESC), ',', 1) AS some_column,
  SUBSTRING_INDEX(GROUP_CONCAT(another_column ORDER BY date_added DESC), ',', 1) AS another_column
FROM t
GROUP BY whatever

GROUP_CONCAT is an aggregation function which implodes values onto one string. It allows for ORDER BY, which we utilize via ORDER BY date_added DESC so as to implode our desired value first. We then slice up the first token in the string via SUBSTRING_INDEX.

The downside here (apart from making the query quite the frightening appearance) is that your numerical values are transformed into texts. Typically no big deal with SQL, but please be aware.

There's another option where you do a semi-self-join, a much lighter one; you will have to give up usage of index. It's quite long to describe; it still uses GROUP_CONCAT and SUBSTRING_INDEX, but only for the purpose of creating a derived table with only relevant keys. This derived table is then joined with original table. See an example in SQL: selecting top N records per group, another solution.

Include All Columns in SELECT GROUP BY Result – How to Guide

Under the assumptions that you have some columns (say x and y in your example), that you don't know if they are functionally equivalent or not - and that these columns do not have any NULL values (which would complicate things), you can use:

SELECT 
    u, v, w,                    -- the grouping columns

    AVG(z) AS z_avg,            -- the non-functionally dependent

    CASE WHEN MIN(x) = MAX(x)   -- a possibly functionally dependent
      THEN MIN(x) ELSE NULL     -- column
    END AS x,

    CASE WHEN MIN(y) = MAX(y)   -- another one
      THEN MIN(y) ELSE NULL
    END AS y
FROM 
    a
GROUP BY 
    u, v, w ;

If a column is not functionally dependent (with the current data), it will have at least 2 distinct values, so the MIN and MAX will be different and the result will be shown as NULL in that column, in all rows. You can then remove it from the query.

Best Answer

Related Solutions

Mysql – Is it possible to reduce a group to a row based on some criteria

Include All Columns in SELECT GROUP BY Result – How to Guide

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