如何在SQL字符串中放置“ if子句”？

因此，这就是我要在MySQL数据库上执行的操作。

我想要做：

SELECT *
    FROM itemsOrdered
    WHERE purchaseOrder_ID = '@purchaseOrdered_ID'
        AND status = 'PENDING'

如果那不会返回任何行（可以通过）if(dr.HasRows == false)，我现在将UPDATE在purchaseOrder数据库中创建一个：

UPDATE purchaseOrder
    SET purchaseOrder_status = 'COMPLETED'
    WHERE purchaseOrder_ID = '@purchaseOrder_ID'

我怎样才能使这个过程更短一些？

对于您的特定查询，您可以执行以下操作：

UPDATE purchaseOrder
    SET purchaseOrder_status = 'COMPLETED'
    WHERE purchaseOrder_ID = '@purchaseOrder_ID' and
          not exists (SELECT *
                      FROM itemsOrdered WHERE purchaseOrder_ID = '@purchaseOrdered_ID' AND status = 'PENDING'
                     )

但是，我可能会猜测您正在循环使用更高的级别。要设置所有此类值，请尝试以下操作：

UPDATE purchaseOrder
    SET purchaseOrder_status = 'COMPLETED'
    WHERE not exists (SELECT 1
                      FROM itemsOrdered
                      WHERE itemsOrdered.purchaseOrder_ID = purchaseOrder.purchaseOrdered_ID AND
                            status = 'PENDING'
                      limit 1
                     )

您可以使用多表UPDATE语法来实现ANTI-JOIN介于purchaseOrder和之间itemsOrdered：

UPDATE purchaseOrder p LEFT JOIN itemsOrdered i
    ON p.purchaseOrder_ID = i.purchaseOrder_ID
   AND i.status = 'PENDING'
SET    p.purchaseOrder_status = 'COMPLETED'
WHERE  p.purchaseOrder_ID = '@purchaseOrder_ID'
   AND i.purchaseOrder_ID IS NULL

由于MySQL不支持if exists(*Your condition*) (*Write your query*)，因此可以通过编写如下代码来实现“ if子句”：

(*Write your insert or update query*) where not exists (*Your condition*)

您还可以使用以下查询来检查记录是否存在，然后对其进行更新：

if not exists(select top 1 fromFROM itemsOrdered
    WHERE purchaseOrder_ID = '@purchaseOrdered_ID'
        AND status = 'PENDING' )
Begin

UPDATE purchaseOrder 
    SET purchaseOrder_status = 'COMPLETED'
    WHERE purchaseOrder_ID = '@purchaseOrder_ID

End

Select FROM t1
    WHERE s11 > ANY
        (SELECT col1,col2 FROM t2
            WHERE NOT EXISTS
                (SELECT * FROM t3
                    WHERE ROW(5*t2.s1,77)=
                        (SELECT 50,11*s1 FROM t4 UNION SELECT 50,77 FROM
                            (SELECT * FROM t5) AS t5)));

if not exists(select top 1 fromFROM itemsOrdered
    WHERE purchaseOrder_ID = '@purchaseOrdered_ID'
        AND status = 'PENDING' )
Begin

UPDATE purchaseOrder 
    SET purchaseOrder_status = 'COMPLETED'
    WHERE purchaseOrder_ID = '@purchaseOrder_ID

End

sql server 2008提供Merge基于单个match语句的插入，更新和删除操作后，也允许您加入。下面的示例示例可能会对您有所帮助。

MERGE Target AS T
USING Source AS S
ON (T.EmployeeID = S.EmployeeID) 
WHEN NOT MATCHED BY TARGET AND S.EmployeeName LIKE 'S%' 
    THEN INSERT(EmployeeID, EmployeeName) VALUES(S.EmployeeID, S.EmployeeName)
WHEN MATCHED 
    THEN UPDATE SET T.EmployeeName = S.EmployeeName
WHEN NOT MATCHED BY SOURCE AND T.EmployeeName LIKE 'S%'
    THEN DELETE 
OUTPUT $action, inserted.*, deleted.*;

这样，您可以在一个语句中插入，更新和删除。

有关更多信息，请参阅https://technet.microsoft.com/zh-cn/library/bb522522(v=sql.105).aspx上的正式文档

如果表包含数百万条记录，则以下查询将快速运行。

UPDATE PO
SET PO.purchaseOrder_status = 'COMPLETED'
FROM purchaseOrder PO
LEFT OUTER JOIN itemsOrdered IOD ON IOD.purchaseOrder_ID = PO.purchaseOrdered_ID and IOD.status = 'PENDING'
WHERE IOD.purchaseOrder_ID IS NULL

您可以声明一个变量，该变量保存选择查询中返回的结果数。如果此变量大于0，则可以运行update语句。

    Declare @ResultCount int
    SELECT @ResultCount = count(*) FROM itemsOrdered WHERE purchaseOrder_ID = '@purchaseOrdered_ID' AND status = 'PENDING'        
    If @ResultCount > 0
UPDATE purchaseOrder SET purchaseOrder_status = 'COMPLETED' WHERE purchaseOrder_ID = '@purchaseOrder_ID'