SQL中PIVOT函数的用法小结

-- 从子查询中选择数据，子查询从源表中选择需要的数据  
SELECT ...    
FROM     
   (  
      -- 源查询，从源表中选取你希望进行PIVOT操作的列  
      SELECT ...    
      FROM <source_table>  
      -- 可以包含WHERE子句、GROUP BY子句等以筛选或组织数据  
   ) AS SourceTable    
  
-- PIVOT操作，将行数据转换为列数据  
PIVOT    
   (  
      -- 聚合函数，用于计算每个新列的值  
      aggregate_function(<column_value>)    
      -- 指定要进行转换的列名  
      FOR <column_name>     
      -- 指定转换后新列的名称列表  
      IN ([first_pivoted_column], [second_pivoted_column], ...)  
   ) AS PivotTable;

CREATE TABLE sales (  
    year INT,  
    product VARCHAR(50),  
    amount DECIMAL(10, 2)  
);  
  
INSERT INTO sales (year, product, amount) VALUES  
(2020, 'A', 100),  
(2020, 'B', 200),  
(2021, 'A', 150),  
(2021, 'B', 250);

SELECT *  
FROM (  
    SELECT year, product, amount  
    FROM sales  
) AS source_table  
PIVOT (  
    SUM(amount)  
    FOR product IN ([A], [B])  
) AS pivot_table;

SELECT year,  
       SUM(CASE WHEN product = 'A' THEN amount ELSE 0 END) AS 'A',  
       SUM(CASE WHEN product = 'B' THEN amount ELSE 0 END) AS 'B'  
FROM sales  
GROUP BY year;

SELECT year,  
       SUM(CASE WHEN product = 'A' THEN amount ELSE 0 END) AS "A",  
       SUM(CASE WHEN product = 'B' THEN amount ELSE 0 END) AS "B"  
FROM sales  
GROUP BY year;