SQL 查询：遍历具有相同值的行并计算出现次数

Question

我有以下表结构：

Name     Fruit
--------------------
Bob      Apple
Bob      Pear
Alice    Apple
Alice    Orange
Alice    Orange
Bob      Orange
Bob      Apple
Alice    Pear
Bob      Orange
Bob      Apple

我想遍历 Name 列，并计算不同 Fruits 的数量。 首选输出将是：

Name    Apple     Pear      Orange
----------------------------------
Bob     3         1         2
Alice   1         1         2

有谁知道我如何在 MySQL 工作台中将其编写为 SQL 查询？

Answer 1

使用条件聚合：

select name,
       sum(fruit = 'Apple') as num_apples,
       sum(fruit = 'Pear') as num_pears,
       sum(fruit = 'Orange') as num_oranges
from t
group by name;

Answer 2

您也可以使用 CASE 表达式

SELECT Name,
       SUM(CASE WHEN fruit = 'Apple' THEN 1 ELSE 0 END) Apples,
       SUM(CASE WHEN fruit = 'Pear' THEN 1 ELSE 0 END) Pear,
       SUM(CASE WHEN fruit = 'Orange' THEN 1 ELSE 0 END) Orange
FROM Table1
GROUP BY Name;

Answer 3

请尝试使用以下 SQL。 “水果”是表的名称：

SET @sql = NULL;
SELECT
  GROUP_CONCAT(DISTINCT
    CONCAT(
      'SUM(IF(tf.Fruit = ''',
      Fruit,
      ''', 1, 0)) AS ',
      Fruit
    )
  ) INTO @sql
FROM fruits;

SET @sql = CONCAT('SELECT tf.name, ', 
                   @sql, 
                  ' FROM fruits tf                                     
                    GROUP BY tf.name');

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