从另一个表的行批量更新表

2桌:

Employees
- EmployeeID
- LeadCount


Leads
- leadID
- employeeID

我想Employees.LeadCount通过计算Leads表中具有相同的引导数来更新列EmployeeID.

注意:可能有超过1个具有相同employeeID的潜在客户,所以我必须做一个DISTINCT(SUM(employeeID)).

UPDATE
    Employees E
SET
    E.LeadCount = (
        SELECT COUNT(L.EmployeeID)
        FROM Leads L
        WHERE L.EmployeeID = E.EmployeeID
    )

您正在为数据同步问题做好准备.在插入,更新或删除Leads表中的行时,您需要不断更新Employees.LeadCount列.

最好的解决方案是根本不存储LeadCount列,而是在需要值时使用SQL聚合查询重新计算潜在客户数.这样它总是正确的.

SELECT employeeID, COUNT(leadId) AS LeadCount
FROM Leads
GROUP BY employeeID;

另一种解决方案是在Leads表上为INSERT,UPDATE和DELETE创建触发器,以便始终使Employees.LeadCount列保持最新.例如,使用MySQL触发器语法:

CREATE TRIGGER leadIns AFTER INSERT ON Leads
FOR EACH ROW BEGIN
  UPDATE Employees SET LeadCount = LeadCount + 1 WHERE employeeID = NEW.employeeID;
END

CREATE TRIGGER leadIns AFTER UPDATE ON Leads
FOR EACH ROW BEGIN
  UPDATE Employees SET LeadCount = LeadCount - 1 WHERE employeeID = OLD.employeeID;
  UPDATE Employees SET LeadCount = LeadCount + 1 WHERE employeeID = NEW.employeeID;
END

CREATE TRIGGER leadIns AFTER DELETE ON Leads
FOR EACH ROW BEGIN
  UPDATE Employees SET LeadCount = LeadCount - 1 WHERE employeeID = OLD.employeeID;
END

如果您使用MySQL,另一个选择是使用多表UPDATE语法.这是SQL的MySQL扩展,它不能移植到其他品牌的RDBMS.首先,将所有行中的LeadCount重置为零,然后对Leads表进行连接,并在连接生成的每一行中增加LeadCount.

UPDATE Employees SET LeadCount = 0;
UPDATE Employees AS e JOIN Leads AS l USING (employeeID)
  SET e.LeadCount = e.LeadCount+1;

联接对更新（和删除）的作用相同，就像对选择（编辑：在某些流行的RDBMS中，至少*）一样：

UPDATE Employees SET
  LeadCount = Leads.LeadCount
FROM Employee
JOIN (
  SELECT EmployeeId, COUNT(*) as LeadCount 
  FROM Leads 
  GROUP BY EmployeeId
) as Leads ON
  Employee.EmployeeId = Leads.EmployeeId  

SUM（DISTINCT EmployeeId）没有意义-您只需要一个COUNT（*）。

MS SQL Server 像MySql一样支持UPDATE ... FROM和DELETE ... FROM语法，但SQL-92标准不支持。SQL-92将让您使用行表达式。我知道DB2支持此语法，但不确定其他语法。坦率地说，我发现SQL-92版本令人困惑-但是标准和理论上的疑问会指出FROM语法违反了关系理论，并且使用不精确的JOIN子句或切换RDBMS供应商时可能导致不可预测的结果。