批量从另一个表更新从行的表

Question

2表：

Employees 
- EmployeeID 
- LeadCount 


Leads 
- leadID 
- employeeID 

我想通过计数Leads表具有相同EmployeeID引线＃更新Employees.LeadCount列。

注意：可能有超过1名领导具有相同的employeeID，所以我必须做一个DISTINCT(SUM(employeeID))。

Answer 1

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

Answer 2

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

Answer 3

从上方钢化和除去从属子查询。

// 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 

编辑它的“分组依据”不“不同”：B（感谢马克·布拉克特）

Answer 4

连接的工作原理相同的更新（和删除），就像他们的选择（编辑做的：在一些流行的RDBMS”，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 

的SUM（DISTINCT雇员）是没有意义的 - 你只需要一个COUNT（*）。

• MS SQL Server支持UPDATE...FROM，和DELETE...FROM语法一样，MySql，但SQL-92标准没有。 SQL-92会让你使用行表达式。我知道DB2支持这种语法，但不确定其他语言。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士会认为，FROM语法违反了关系理论，并且可能导致无法预测的结果，但不准确的JOIN子句或切换RDBMS供应商。

Answer 5

您正在设置自己的数据同步问题。随着Leads表中的行被插入，更新或删除，您需要不断更新Employees.LeadCount列。

最好的解决方案不是存储LeadCount列，而是根据需要重新计算带有SQL聚合查询的销售线索数。这样它总是正确的。

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

另一个解决办法是对信息表的INSERT，UPDATE创建触发器和DELETE，让你保持Employees.LeadCount列当前所有的时间。例如，使用MySQL触发语法：

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 

如果您使用的是MySQL，另一种选择是使用多表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表执行联接，并在联接所产生的每行中增加LeadCount。

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