Question

我有一个MySQL语句，用于查询联系人表。当它们被加载到我的DataGrid中时，同一个联系人有多个副本（表中只有1个）。

这让我相信我的MySQL陈述有问题。这是我到目前为止所拥有的;

myQuery.CommandText = @"SELECT contacts.contactID, contacts.companyID, 
companies.name, contacts.donestatus, employees.name,
people.ID, people.firstname, people.lastname, 
contacts.contractID, contacts.date, contacts.time, 
presets.presettext, contacts.madeby, contacts.description
FROM contacts
LEFT OUTER JOIN companies ON contacts.companyID = companies.ID
LEFT OUTER JOIN employees ON contacts.employeeID = employees.ID
LEFT OUTER JOIN people ON contacts.personID = people.ID
JOIN presets ON contacts.type = presets.presetIDFoxPro
WHERE contacts.companyid = @CompanyID
AND presets.presetreferencefoxpro = 8
ORDER BY contacts.date DESC";

我检查了方法，它肯定只调用一次，所以对我来说它必须是声明。多个JOINS是否会为每个成功的Contact加载JOIN？

Answer 1

在这种情况下，它取决于您想要出现的内容。如果联系人可以拥有多个相关公司，员工或人员，那么您将使用当前SQL将其中的每个组合带回记录。例如，如果一个联系人有2个，那么8行（即2 x 2 x 2）会回来进行该联系。

如果您希望每个联系人在一行上显示所有详细信息，那么您可以使用子查询来获取每个联系人的所有联接详细信息，然后将其结果与联系人表联系起来。

举个简单的例子： -

SELECT contacts.contactID, contacts.companyID, 
        sub_companies.name, contacts.donestatus, sub_employees.name,
        sub_people.all_id, sub_people.all_firstname, sub_people.all_lastname, 
        contacts.contractID, contacts.date, contacts.time, 
        presets.presettext, contacts.madeby, contacts.description
FROM contacts
INNER JOIN presets ON contacts.type = presets.presetIDFoxPro
LEFT OUTER JOIN 
(
    SELECT ID, GROUP_CONCAT(name)
    FROM companies 
    GROUP BY ID
) sub_companies ON contacts.companyID = sub_companies.ID
LEFT OUTER JOIN 
(
    SELECT ID, GROUP_CONCAT(name)
    FROM employees 
    GROUP BY ID
) sub_employees ON contacts.employeeID = sub_employees.ID
LEFT OUTER JOIN 
(
    SELECT ID, GROUP_CONCAT(ID) AS all_id, GROUP_CONCAT(firstname) AS all_firstname, GROUP_CONCAT(lastname) AS all_lastname
    FROM people 
    GROUP BY ID
) sub_people ON contacts.personID = sub_people.ID
WHERE contacts.companyid = @CompanyID
AND presets.presetreferencefoxpro = 8
ORDER BY contacts.date DESC

MySQL加入返回多行

1 个答案: