如何计算SQL中唯一列和行的出现次数？

Question

This question is very similar to : https://stackoverflow.com/posts/1503959/edit But I want to take it one step further, and create new columns for each age, with a count for that age.

I have a table of people:

name  | age
--------
bill  | 25
tom   | 25
tom   | 23
tom   | 23

I want to query for all names, and additional columns that counts how many names are of the same age:

name  | 23 | 25
----------------
bill  | 0  | 1
tom   | 2  | 1

What's the most efficient way of doing this? I fear that a sub-query will be slow, and I'm wondering if there's a better way . Is there?

Answer 1

In case you have many age values,dynamic pivot is very useful

SELECT
  GROUP_CONCAT(DISTINCT
    CONCAT(
      'max(case when age = ''',
      age,
      ''' then 1 else 0 end) AS ','`',
      age,'`'
    ) ORDER BY age
  ) INTO @sql
FROM
  t;

SET @sql = CONCAT('SELECT name, ', @sql, ' 
                  FROM t 
                   GROUP BY name');

PREPARE stmt FROM @sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;

FIDDLE

Answer 2

Easiest way how to do this in SQL is using GROUP BY .

SELECT name, age, COUNT(*) AS occurence FROM table GROUP BY name, age

But you get data in different format. You need to reformat them in PHP, because SQL doesn't work way you want.