Question

2张表：

Employees
- EmployeeID
- LeadCount


Leads
- leadID
- employeeID

我想通过计算Employees.LeadCount表中具有相同Leads的潜在客户数来更新EmployeeID列。

注意：同一个employeeID可能有多个潜在客户，因此我必须执行DISTINCT(SUM(employeeID))。

Answer 1

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

Answer 2

您正在为自己设置数据同步问题。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，以便始终保持Employees.LeadCount列的最新状态。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;

Answer 3

加入更新（和删除）的工作方式与选择相同（编辑：在一些流行的RDBMS中，至少*）：

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）毫无意义 - 你只需要一个COUNT（*）。

MS SQL Server支持UPDATE...FROM和DELETE...FROM语法，MySql也支持，但SQL-92标准不支持。 SQL-92会让你使用行表达式。我知道DB2支持这种语法，但不确定是否有其他语法。坦率地说，我发现SQL-92版本令人困惑 - 但标准和理论方面的人会争辩说，FROM语法违反了关系理论，并且可能导致不可预测的结果，不精确的JOIN子句或切换RDBMS供应商。

Answer 4

UPDATE Employees SET LeadCount = (
  SELECT Distinct(SUM(employeeID)) FROM Leads WHERE Leads.employeeId = Employees.employeeId
)

Answer 5

从上面偷窃并删除从属子查询。

// create tmp -> TBL (EmpID, count)

insert into TBL 
   SELECT employeeID COUNT(employeeID) Di
   FROM Leads WHERE Leads.employeeId = Employees.employeeId GROUP BY EmployeeId
UPDATE Employees SET LeadCount = (
  SELECT count FROM TBL WHERE TBL.EmpID = Employees.employeeId
)

// drop TBL

编辑这是“group By”而不是“distinct”：b（感谢Mark Brackett）

从另一个表的行批量更新表

5 个答案: