PIVOT 子句
描述
PIVOT 子句用于数据透视。我们可以根据特定的列值获取聚合值，这些值将转换为 SELECT 子句中使用的多个列。PIVOT 子句可以在表名或子查询之后指定。
语法
PIVOT ( { aggregate_expression [ AS aggregate_expression_alias ] } [ , ... ]
FOR column_list IN ( expression_list ) )
参数
-
aggregate_expression
指定聚合表达式（SUM(a)、COUNT(DISTINCT b) 等）。
-
aggregate_expression_alias
指定聚合表达式的别名。
-
column_list
包含
FROM子句中的列，用于指定我们要替换为新列的列。我们可以使用括号将列括起来，例如(c1, c2)。 -
expression_list
指定新列，用于匹配
column_list中的值作为聚合条件。我们还可以为它们添加别名。
示例
CREATE TABLE person (id INT, name STRING, age INT, class INT, address STRING);
INSERT INTO person VALUES
(100, 'John', 30, 1, 'Street 1'),
(200, 'Mary', NULL, 1, 'Street 2'),
(300, 'Mike', 80, 3, 'Street 3'),
(400, 'Dan', 50, 4, 'Street 4');
SELECT * FROM person
PIVOT (
SUM(age) AS a, AVG(class) AS c
FOR name IN ('John' AS john, 'Mike' AS mike)
);
+------+-----------+---------+---------+---------+---------+
| id | address | john_a | john_c | mike_a | mike_c |
+------+-----------+---------+---------+---------+---------+
| 200 | Street 2 | NULL | NULL | NULL | NULL |
| 100 | Street 1 | 30 | 1.0 | NULL | NULL |
| 300 | Street 3 | NULL | NULL | 80 | 3.0 |
| 400 | Street 4 | NULL | NULL | NULL | NULL |
+------+-----------+---------+---------+---------+---------+
SELECT * FROM person
PIVOT (
SUM(age) AS a, AVG(class) AS c
FOR (name, age) IN (('John', 30) AS c1, ('Mike', 40) AS c2)
);
+------+-----------+-------+-------+-------+-------+
| id | address | c1_a | c1_c | c2_a | c2_c |
+------+-----------+-------+-------+-------+-------+
| 200 | Street 2 | NULL | NULL | NULL | NULL |
| 100 | Street 1 | 30 | 1.0 | NULL | NULL |
| 300 | Street 3 | NULL | NULL | NULL | NULL |
| 400 | Street 4 | NULL | NULL | NULL | NULL |
+------+-----------+-------+-------+-------+-------+