Question

    CREATE TABLE Institutions
    (
    Institution_ID INTEGER PRIMARY KEY,
    Institution_Name VARCHAR(200))

CREATE TABLE Contact_Persons
(
Contact_No INTEGER PRIMARY KEY,
First_Name VARCHAR(60) NOT NULL,
Last_Name VARCHAR(60) NOT NULL,
Institution_ID INTEGER,
FOREIGN KEY (Institution_ID) REFERENCES Institutions(Institution_ID))

如何编写一个mysql查询，显示拥有多个联系人的机构？查询应显示表机构中的Institution_ID和Institution_Name。以下查询有效，但仅显示Institution_ID。我也希望看到Institution_Name。我认为这两个表之间必须有一个连接。请帮忙。

SELECT Institution_ID
FROM Contact_Persons
GROUP BY Institution_ID
HAVING COUNT(*)>1;

Answer 1

试试这个

SELECT I.Institution_ID,I.Institution_name
FROM Contact_Persons as C inner join Institutions as I on 
C.Institution_ID=I.Institution_ID
GROUP BY I.Institution_ID,I.Institution_name
HAVING COUNT(*)>1;

Answer 2

你是对的，你只需要加入你的两个表，并确保你也按照你想要选择的所有内容进行分组（除非它是一个聚合），所以在这种情况下，将Institution_Name添加到组中子句：

SELECT i.Institution_ID, i.Institution_Name
FROM Contact_Persons AS cp
    INNER JOIN Institutions AS i
        ON i.Institution_ID = cp.Institution_ID
GROUP BY i.Institution_ID, i.Institution_Name
HAVING COUNT(*) > 1;

如何编写一个查询，显示有多个联系人的机构？

2 个答案: