mysql - mysql中的交叉表

Question

我有这个数据

| car_name | year | price 
| Honda    | 2011 |  123
| Honda    | 2012 |  124
| Suzuki   | 2011 |  1234
| Suzuki   | 2012 |  1235

我将如何将其更改为

| car_name | 2011 | 2012 |
| Honda    | 123  | 124
| Suzuki   | 1234 | 1235

请帮我

Answer 1

在 MySQL 中，您可以执行以下操作：

SELECT car_name,
  max(case when year = '2011' then price ELSE 0 END) as `2011`,
  max(case when year = '2012' then price ELSE 0 END) as `2012`
FROM t
GROUP BY car_name

请参阅带有演示的 SQL Fiddle

Answer 2

在 PHP 中，这是您的答案：

$sql = "SELECT DISTINCT `year` FROM `table`";
$result = mysql_query($sql);


$sql2_select = "SELECT t1.`car_name`";
$sql2_from = "FROM `table` t1";

$i = 2;
while($row = mysql_fetch_row($result)) {
  $year = $row[0];

  $sql2_select .= ", t{$i}.`price` AS `$year`"

  $sql_from .= " LEFT JOIN (SELECT `car_name`, `price` WHERE `year` = '$year') AS t{$i}";
  $sql_from .= " ON t1.`car_name` = t2.`car_name`";

  $i++;
}

$sql2 = $sql2_select . ' ' . $sql2_from . ' ' . "GROUP BY t1.car_name";

$result = mysql_query($sql2);

Answer 3

在 MySQL 中进行交叉表的一种方法是使用子选择：

select car_name, 
(select price from t t2 where t2.car_name = t1.car_name and year = 2011) as '2011',
(select price from t t2 where t2.car_name = t1.car_name and year = 2012) as '2012'
from t t1
group by car_name

SQL小提琴

如果您每年每辆车有多个记录，那么 sum(price)。