Question

我目前正在学习mysql中的子查询，只想弄清楚im是否在正确的轨道上。下面的代码确实带来了结果，但是不确定是否正确执行了。问题是

“使用IN语句查找在1997年4月提交了至少一份订单的所有客户的代码和公司名称。”

    uglifyOptions: {
          compress: {
                warnings: false,
                pure_funcs: ['console.log']
          }
    }

我仔细检查了插入表中的值，即使产生了结果，它似乎也不正确。

（注意，使用EXIST子句与IN子句相同，但不是WHERE CUSTOMER_CODE IN而是WHERE EXISTS吗？

我尝试过

SELECT CUSTOMER_CODE, COMPANY_NAME
FROM CUSTOMER
WHERE CUSTOMER_CODE IN (SELECT CUSTOMER_CODE
     FROM ORDERS
        WHERE ORDER_DATE BETWEEN  '1997-04-01' AND '1997-04-30'
        GROUP BY CUSTOMER_CODE )

这是我正在使用的两个标签

SELECT CUSTOMER_CODE, COMPANY_NAME
FROM CUSTOMER
WHERE NOT EXISTS (SELECT CUSTOMER_CODE
     FROM ORDERS
        WHERE ORDER_DATE BETWEEN  '1997-04-01' AND '1997-04-30'
        GROUP BY CUSTOMER_CODE );

CREATE TABLE CUSTOMER
(
    CUSTOMER_CODE   VARCHAR(5)  NOT NULL,
    COMPANY_NAME    VARCHAR(40) NOT NULL,
    CONTACT_NAME    VARCHAR(30),
    CONTACT_TITLE   VARCHAR(30),
    ADDRESS         VARCHAR(60),
    CITY        VARCHAR(15),
    REGION      VARCHAR(15),
    POSTAL_CODE     VARCHAR(10),
    COUNTRY         VARCHAR(15),
    PHONE       VARCHAR(24),
    FAX         VARCHAR(24),
    CONSTRAINT PK_CUSTOMER PRIMARY KEY (CUSTOMER_CODE)
);

但我收到一个空集谢谢大家的时间

Answer 1

代替IN子句，您可以基于同一子查询使用INNER JOIN ..

通常比In子句

效果更好

  SELECT CUSTOMER_CODE, COMPANY_NAME
  FROM CUSTOMER
  INNER JOIN (
      SELECT DISTINCT CUSTOMER_CODE
      FROM ORDERS
      WHERE ORDER_DATE BETWEEN  '1997-04-01' AND '1997-04-30'
  ) t on t.CUSTOMER_CODE = CUSTOMER.CUSTOMER_CODE

当您不使用聚合功能时，您不应使用group by for（默认情况下，在最新版本的mysql中这无效）如果只需要不同的结果，请使用DISTINCT

Answer 2

您必须使用EXISTS而不是NOT EXISTS：

SELECT c.CUSTOMER_CODE, c.COMPANY_NAME
FROM CUSTOMER c
WHERE EXISTS (
  SELECT CUSTOMER_CODE FROM ORDERS
  WHERE 
  CUSTOMER_CODE = c.CUSTOMER_CODE
  AND
  ORDER_DATE BETWEEN '1997-04-01' AND '1997-04-30'
);

您还可以通过以下方法查看ORDER_DATE：

YEAR(ORDER_DATE) = 1997 AND MONTH(ORDER_DATE) = 4

您也可以使用IN：

SELECT CUSTOMER_CODE, COMPANY_NAME
FROM CUSTOMER
WHERE CUSTOMER_CODE IN (
  SELECT CUSTOMER_CODE FROM ORDERS
  WHERE YEAR(ORDER_DATE) = 1997 AND MONTH(ORDER_DATE) = 4 
);

如果要查找未提交订单的所有客户总数：

SELECT COUNT(DISTINCT c.CUSTOMER_CODE) AS COUNTER
FROM CUSTOMER c
WHERE NOT EXISTS (
  SELECT CUSTOMER_CODE FROM ORDERS
  WHERE CUSTOMER_CODE = c.CUSTOMER_CODE
)

在子查询中使用count

2 个答案: