[英]Bulk updating a table from rows from another table
2桌:
Employees
- EmployeeID
- LeadCount
Leads
- leadID
- employeeID
我想通過計算Leads
表中具有相同EmployeeID
的潛在客戶數來更新Employees.LeadCount
列。
注意:可能有多個具有相同employeeID的潛在客戶,因此我必須執行DISTINCT(SUM(employeeID))
。
UPDATE
Employees E
SET
E.LeadCount = (
SELECT COUNT(L.EmployeeID)
FROM Leads L
WHERE L.EmployeeID = E.EmployeeID
)
您正在為數據同步問題做好准備。 在插入,更新或刪除Leads表中的行時,您需要不斷更新Employees.LeadCount列。
最好的解決方案是根本不存儲LeadCount列,而是在需要值時使用SQL聚合查詢重新計算潛在客戶數。 這樣它總是正確的。
SELECT employeeID, COUNT(leadId) AS LeadCount
FROM Leads
GROUP BY employeeID;
另一種解決方案是在Leads表上為INSERT,UPDATE和DELETE創建觸發器,以便始終使Employees.LeadCount列保持最新。 例如,使用MySQL觸發器語法:
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,另一個選擇是使用多表UPDATE語法。 這是SQL的MySQL擴展,它不能移植到其他品牌的RDBMS。 首先,將所有行中的LeadCount重置為零,然后對Leads表進行連接,並在連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;
連接的工作方式與更新(和刪除)相同,就像它們對選擇一樣(編輯: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)毫無意義 - 你只需要一個COUNT(*)。
UPDATE Employees SET LeadCount = (
SELECT Distinct(SUM(employeeID)) FROM Leads WHERE Leads.employeeId = Employees.employeeId
)
從上面進行鋼化並刪除從屬子查詢。
// 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
編輯它是“group By”而不是“distinct”:b(感謝Mark Brackett)
聲明:本站的技術帖子網頁,遵循CC BY-SA 4.0協議,如果您需要轉載,請注明本站網址或者原文地址。任何問題請咨詢:yoyou2525@163.com.