Is there a way to SELECT and UPDATE rows at the sa

问题:

I'd like to update a set of rows based on a simple criteria and get the list of PKs that were changed. I thought I could just do something like this but am worried about possible concurrency problems:

SELECT Id FROM Table1 WHERE AlertDate IS NULL;
UPDATE Table1 SET AlertDate = getutcdate() WHERE AlertDate IS NULL;

If that is wrapped in a transaction are there any concurrency issues that can occur? Or is there a better way to do this?

回答1:

Consider looking at the OUTPUT clause capability of UPDATE (also DELETE and INSERT). Example from the linked MSDN page:

UPDATE TOP (10) HumanResources.Employee
SET VacationHours = VacationHours * 1.25,
    ModifiedDate = GETDATE() 
OUTPUT inserted.BusinessEntityID,
       deleted.VacationHours,
       inserted.VacationHours,
       inserted.ModifiedDate
INTO @MyTableVar;

回答2:

One way to handle this is to do it in a transaction, and make your SELECT query take an update lock on the rows selected until the transaction completes.

BEGIN TRAN

SELECT Id FROM Table1 WITH (UPDLOCK)
WHERE AlertDate IS NULL;

UPDATE Table1 SET AlertDate = getutcdate() 
WHERE AlertDate IS NULL;

COMMIT TRAN 

This eliminates the possibility that a concurrent client updates the rows selected in the moment between your SELECT and your UPDATE.

When you commit the transaction, the update locks will be released.

Another way to handle this is to declare a cursor for your SELECT with the FOR UPDATE option. Then UPDATE WHERE CURRENT OF CURSOR. The following is not tested, but should give you the basic idea:

DECLARE cur1 CURSOR FOR
  SELECT AlertDate FROM Table1 
  WHERE AlertDate IS NULL
  FOR UPDATE;

DECLARE @UpdateTime DATETIME

SET @UpdateTime = GETUTCDATE()

OPEN cur1;

FETCH NEXT FROM cur1;

WHILE @@FETCH_STATUS = 0
BEGIN

  UPDATE Table1 AlertDate = @UpdateTime
  WHERE CURRENT OF cur1;

  FETCH NEXT FROM cur1;

END

回答3:

It'd be easier to do your UPDATE first and then run 'SELECT ID FROM INSERTED'.

Take a look at SQL Tips for more info and examples.

回答4:

Many years later...

The accepted answer of using the OUTPUT clause is good. I had to dig up the actual syntax, so here it is:

DECLARE @UpdatedIDs table (ID int)
UPDATE 
    Table1 
SET 
    AlertDate = getutcdate() 
OUTPUT
    inserted.Id
INTO
    @UpdatedIDs
WHERE 
    AlertDate IS NULL;

ADDED SEP 14, 2015:

"Can I use a scalar variable instead of a table variable?" one may ask... Sorry, but no you can't. You'll have to SELECT @SomeID = ID from @UpdatedIDs if you need a single ID.

回答5:

Perhaps something more like this?

declare @UpdateTime datetime

set @UpdateTime = getutcdate()

update Table1 set AlertDate = @UpdateTime where AlertDate is null

select ID from Table1 where AlertDate = @UpdateTime

回答6:

I have faced the same issue; I have to update the credit amount, and have to get modified time, along with credit details from DB. It is basically

SYNCHRONOUSLY/ATOMICALLY perform (UPDATE then GET) in MYSQL

I have tried many options and found one that solved my issue.

1) OPTION_1 SELECT FOR UPDATE

This is maintaining the lock till update (SYNC from GET to UPDATE), but i need lock after update till the GET.

2) OPTION_2 Stored procedure

Stored procedure will not execute synchronously like redis lua, So there also we need sync code to perform that.

3) OPTION_3 Transaction

I have used JPA entityManager like below, thought that before commit no one can update, and before commit i will get the updated object along with modified time (from DB). But i didn't get the latest object. Only commit i got the latest.

    try {
        entityManager.getTransaction().begin();
        //entityManager.persist(object);
        int upsert = entityManager.createNativeQuery(
        "update com.bill.Credit c set c.balance = c.balance - ?1
          where c.accountId = ?2 and c.balance >= ?1").executeUpdate(); 
             //c.balance >= ? for limit check
        Credit newCredit = entityManager.find(Credit.class, "id");
        entityManager.refresh(newCredit); //SHOULD GET LATEST BUT NOT
        entityManager.getTransaction().commit();
    } finally {     
        entityManager.unwrap(Session.class).close();
    } 

4) OPTION_4 LOCK solved the issue, so before update i acquired the lock; then after GET i have released the lock.

private Object getLock(final EntityManager entityManager, final String Id){

    entityManager.getTransaction().begin();
    Object obj_acquire = entityManager.createNativeQuery("SELECT GET_LOCK('" + Id + "', 10)").getSingleResult();
    entityManager.getTransaction().commit();
    return obj_acquire;
}


private Object releaseLock(final EntityManager entityManager, final String Id){

    entityManager.getTransaction().begin();
    Object obj_release = entityManager.createNativeQuery("SELECT RELEASE_LOCK('" + Id + "')").getSingleResult();
    entityManager.getTransaction().commit();
    return obj_release;
}

回答7:

if it's inside the transaction, the database locking system will take care of concurrency issues. of course, if you use one (the mssql default is that it uses lock, so it states if you don't override that)

回答8:

in SQL 2008 a new TSQL statement "MERGE" is introduced which performs insert, update, or delete operations on a target table based on the results of a join with a source table. You can synchronize two tables by inserting, updating, or deleting rows in one table based on differences found in the other table.

http://blogs.msdn.com/ajaiman/archive/2008/06/25/tsql-merge-statement-sql-2008.aspx http://msdn.microsoft.com/en-us/library/bb510625.aspx

回答9:

Edit: my bad, you wanted the select to show results after the update, not update from a select.

Have you tried a sub-select?

update mytable set mydate = sysdate 
where mydate in (select mydate from mytable where mydate is null);