Batches, Scripts, Transactions-SQL

Server 7

A batch is a set of Transact-SQL statements that are interpreted together by SQL Server.

They are submitted together, and the end of the batch is detected by usage of the keyword GO.

SELECT au_id, au_lname FROM authors

SELECT pub_id, pub_name FROM publishers

INSERT INTO publishers VALUES (`9998','SAMS Publishing', 'Seattle', 'WA','USA')

GO

Batches

Batches follow several rules.

All of the SQL statements are compiled together.

If there is a error anywhere in the batch, the entire batch is cancelled.

If you were to modify the previous set of SQL statements and introduce an error, you would get an error message from SQL Server, and none of the statements would run.

For example:

SELECT au_id, au_lname FROM authors

SELECT pub_id, pub_name FROM publishers

INSERT INTO notable VALUES (`9998','SAMS Publishing', 'Seattle', 'WA','USA')

GO

Msg 208, Level 16, State 1 Invalid object name `notable‘

There is no such database object named notable.

Some statements can be combined in a batch, while other statements are restricted.

The following CREATE statements can be bound together within a single batch.

•CREATE DATABASE •CREATE TABLE •CREATE INDEX

These CREATE statements cannot be combined with others.

•CREATE RULE •CREATE TRIGGER •CREATE PROCEDURE •CREATE DEFAULT •CREATE VIEW

There are some additional rules for batches.

•Check constraints can't be bound and used with tables in the same batch.

•You cannot drop and then re-create an object with the same name within a single batch.

•You cannot alter a table and then use the new columns within the same batch.

•SET statements take effect only at the beginning of the next batch. Because of this, it's a good idea to always follow a SET statement with the keyword GO.

•There is a limit of 128KB for the size of a batch. This limit doesn't apply to WRITETEXT and UPDATETEXT statements.

A CREATE TABLE with a check constraint can be in a batch, but if you attempt to insert data that would require checking, the constraint is not enforced.

Data that violates the check constraint, will be inserted anyway. An insert in a new batch will produce the expected error, however.

The following statements would fail with the error that table2 already exists in the database.

DROP TABLE table2

CREATE TABLE table2 (col1 INT NOT NULL)

A script is simply a set of one or more batches.

Scripts typically are executed as part of some unit of work that needs to be accomplished, such as a data load or database maintenance.

Scripts

Example of a script.

SELECT au_id, au_lname FROM authors

SELECT pub_id, pub_name FROM publishers

INSERT publishers VALUES (`9997','SAMS Publishing', 'Seattle', 'WA','USA')

GO

SELECT * FROM stores

GO

DELETE publishers WHERE pub_id = `9997'

GO

Note that batches and scripts don't necessarily have

anything to do with transactions

There are two types of transactions: explicit and implicit.

Explicit transactions are transactions that are manually configured.

Reserved words are used to indicate the beginning and end of explicit transactions.

These reserved words include BEGIN TRANSACTION, COMMIT TRANSACTION, ROLLBACK TRANSACTION, SAVE TRANSACTION.

Transactions

BEGIN TRAN

UPDATE authors SET city = `San Jose' WHERE name = `Smith'

INSERT titles VALUES (`BU1122', 'Teach Yourself SQL Server in 21 days','business','9998',$35.00, $1000.00,10,4501, `A great book!')

SELECT * from titleauthor

COMMIT TRAN

Explicit Transactions

To cancel transactions.

BEGIN TRAN

Delete titles where type = `business'

IF @@ERROR > 0

ROLLBACK TRAN

ELSE

COMMIT TRAN

The ROLLBACK TRAN statement will cancel the transaction completely.

Any work that was done in the transaction up to that point will be rolled back, or canceled.

break points can be placed within a transaction.Selectively roll back to those points.

BEGIN TRAN UPDATE table1 SET col1 = 5 WHERE col2 = 14 SAVE TRAN savepoint1 INSERT table2 values (1401,'book review','a1201',9900) IF @@error > 0 ROLLBACK TRAN savepoint1 DELETE table3 WHERE col1 > 1401 IF @@error > 0 ROLLBACK TRAN ELSE COMMIT TRAN

Example-

The following succeeds in inserting one row into the table, even though there is an error in the transaction (duplicate primary key value)

create procedure r as

declare @p varchar(20);

create table a

(k int,

constraint fg primary key(k));

SELECT @p = 'M';

begin transaction @p;

insert into a values(1);

insert into a values(1);

commit transaction @p;

So, the transaction is responsible for instigating the ROLLBACK

A transaction must specifically request ROLLBACK-it is not automatic.

e.g.

create procedure r as

declare @p varchar(20);

create table a

(k int,

constraint fg primary key(k));

SELECT @p = 'M';

begin transaction @p;

insert into a values(1);

insert into a values(1);

IF @@error > 0 ROLLBACK

commit transaction @p;

Triggers can instigate ROLLBACK’s so the integrity of the database is not dependant on transactions controlling their own ROLLBACK’s

Triggers, Transactions and Rollback

If a ROLLBACK TRANSACTION is issued in a trigger:

·All data modifications made to that point in the current transaction are rolled back, including any that were made by the trigger.

·The trigger continues executing any remaining statements after the ROLLBACK statement. If any of these statements modify data, the modifications are not rolled back. No nested triggers are fired by the execution of these remaining statements.

·The statements in the batch after the statement that fired the trigger are not executed.

ROLLBACK TRANSACTION

statements in stored procedures do not affect subsequent statements in the batch that called the procedure; subsequent statements in the batch are executed.

statements in triggers terminate the batch containing the statement that fired the trigger; subsequent statements in the batch are not executed.

When a constraint is violated in an INSERT, UPDATE, or DELETE statement, the statement is terminated.

However, the transaction (even if the statement is part of an explicit transaction) continues to be processed.

The ROLLBACK TRANSACTION statement is used by checking the @@ERROR system function.

If a table has FOREIGN KEY or CHECK CONSTRAINTS and triggers, the constraint conditions are evaluated before the trigger is executed.

Summary

SQL operations can be encapsulated in batches.

Batches can be grouped into scripts.

Transactions can be produced explicitly.

Great care has to be taken over the rules governing the execution of batches, scripts and transactions.

Transaction are responsible for instigating rollbacks.

Triggers are capable of instigating rollbacks.

Great care has to be taken over the Rules governing the execution of triggers.