sql queries and inserts

开发者_开发百科I have a random question. If I were to do a sql select and while the sql server was querying my request someone else does a insert statement... could that data that was inputted in that insert statement also be retrieved from my select statement?

Queries are queued, so if the SELECT occurs before the INSERT there's no possibility of seeing the newly inserted data.

Using default isolation levels, SELECT is generally given higher privilege over others but still only reads COMMITTED data. So if the INSERT data has not been committed by the time the SELECT occurs--again, you wouldn't see the newly inserted data. If the INSERT has been committed, the subsequent SELECT will include the newly inserted data.

If the isolation level allowed reading UNCOMMITTED (AKA dirty) data, then yes--a SELECT occurring after the INSERT but before the INSERT data was committed would return that data. This is not recommended practice, because UNCOMMITTED data could be subject to a ROLLBACK.

If the SELECT statement is executed before the INSERT statement, the selected data will certainly not include the new inserted data.

What happens in MySQL with MyISAM, the default engine, is that all INSERT statements require a table lock; as a result, once an INSERT statement is executed, it first waits for all existing SELECTs to complete before locking the table, performs the INSERT, and then unlocks it.

For more information, see: Internal Locking Methods in the MySQL manual

No, a SELECT that is already executing that the moment of the INSERT will never gather new records that did not exist when the SELECT statement started executing.

Also if you use the transactional storage engine InnoDB, you can be assured that your SELECT will not include rows that are currently being inserted. That's the purpose of transaction isolation, or the "I" in ACID.

For more details see http://dev.mysql.com/doc/refman/5.1/en/set-transaction.html because there are some nuances about read-committed and read-uncommitted transaction isolation modes.

I don't know particulars for MySQL, but in SQL Server it would depend on if there were any locking hints used, and the default behavior for locks. You have a couple of options:

• Your SELECT locks the table, which means the INSERT won't process until your select is finished.
• Your SELECT is able to do a "dirty read" which means the transaction doesn't care if you get slightly out-of-date data, and you miss the INSERT
• Your SELECT is able to do a "dirty read" but the INSERT happens before the SELECT hits that row, and you get the result that was added.

The only way you do that is with a "dirty read".

Take a look at MYSql's documentation on TRANSACTION ISOLATION LEVELS to get a better understanding of what that is.