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SQL Workshop
Day 3
Copyright © 2011 Accenture All Rights Reserved. Accenture, its logo, and High Performance Delivered are trademarks of Accenture.
Day 3 – SQL
Agenda
• SQL
•
•
•
•
INSERT
UPDATE
DELETE
MERGE
• Database transactions
• Practice
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2
Modifying Data
DML is the subset of SQL that is employed to
change data
INSERT Adds rows to a table
UPDATE Changes the value stored in a table
MERGE Updates or inserts rows from one table into
another
DELETE Removes rows from a table
SELECT FOR UPDATE Prevents other sessions from
performing DML on selected rows
LOCK TABLE Prevents other sessions from
performing DML on a table
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3
INSERT statement
INSERT INTO table [(column1, column2, …)]
VALUES (value1, value2, …);
• To add new rows you have to use INSERT statement
• One row a time is inserted with the syntax
INSERT INTO dept (deptno, dname, loc)
VALUES (40, ‘IT’, ‘Lodz’);
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4
Adding new rows
INSERT
SELECT
FROM
WHERE
INTO table1 [(column1, column2, …)]
value1, value2, …
table2
…;
• To add / copy rows you have to use INSERT - SELECT statement
INSERT INTO dept (deptno, dname, loc)
SELECT 50, ‘BPO’, ‘Warsaw’
FROM DUAL;
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5
UPDATE statement
UPDATE table
SET column1 = value1 [, column2 = value2, …]
[WHERE condition];
• To modify existing rows you have to use UPDATE statement
• Update more than one row at a time if required
UPDATE dept
SET dname = ‘SUPER IT’
WHERE deptno = 40;
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6
DELETE statement
DELETE [FROM] table
[WHERE condition];
• You can remove rows from a table by using DELETE statement
DELETE FROM dept
WHERE deptno = 40;
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7
MERGE statement
• Provides the ability to conditionally update or insert
data into a database table
• Performs UPDATE when the row exists and an
INSERT when it is new row:
– Increases performance
– Useful in data warehouse applications
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8
MERGE statement example
MERGE INTO bonuses B
USING (
SELECT employee_id, salary
FROM employee
WHERE dept_no =20) E
ON (B.employee_id = E.employee_id)
WHEN MATCHED THEN
UPDATE SET B.bonus = E.salary * 0.1
WHEN NOT MATCHED THEN
INSERT (B.employee_id, B.bonus)
VALUES (E.employee_id, E.salary * 0.05);
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9
?Question
Which are DML statements ?
A ) MERGE...
B ) CREATE...
C ) DROP...
D ) DELETE...
E ) COMMIT...
F ) UPDATE...
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10
Database transactions
• A database transactions consists of one of the
following:
– DML statements which constitute one consistent change to the
data
– One DML statement (CREATE, DROP, …)
– One DCL statement (GRANT, REVOKE)
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11
Transaction is ACID
Atomic: all changes to the database made in a transaction are
rolled back if any change fails.
Consistent: the effects of a transaction take the database from one
consistent state to another consistent state.
Isolated: the intermediate steps in a transaction are not visible to
other users of the database.
Durable: when a transaction is completed (committed or rolled
back), its effects persist in the database.
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12
Transaction
begin - end
• Begin when the first DML SQL statement is executed
• End with one of the following events:
–
–
–
–
A COMMIT or ROLLBACK statement is issued
A DDL or DCL statement executes (automatic commit)
The connection is broken (automatic rollback)
The system crashes (automatic rollback)
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13
Rolling back transaction to a marker
• Create a marker in a current transaction by using
SAVEPOINT statement
• Rollback to the marker by using ROLLBACK TO
SAVEPOINT statement
UPDATE …
(10 rows updated)
SAVEPOINT after_update;
INSERT …
(2 rows inserted)
ROLLBACK TO SAVEPOINT after_update;
COMMIT;
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14
?Question
Which of the following commands can be rolled back? (Choose all
correct answers.)
•
•
•
•
•
•
COMMIT
DELETE
INSERT
MERGE
TRUNCATE
UPDATE
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15
Locking rows
• In order to perform some actions it is required to have exclusive access
to particular object (in example we want to edit lines of order and we have
to assure that nobody will approve the order simultaneously) – we have
to lock master object (in this case it is row in table ORDERS)
• In order to lock a row or a group of rows we have to perform
SELECT
FROM
WHERE
FOR
*
table
condition
UPDATE;
• We can also extend the statement with NOWAIT clause
– When it is possible to lock the rows – standard SELECT is performed and rows
are locked
– When it is not possible to lock the rows – exception is thrown: ORA-00054
resource busy and acquire with NOWAIT specified. (It is required to define exception in PL/SQL)
• The lock will be released when the transaction finishes (commit or
rollback)
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16
Locks and waiters
• DML and SELECT FOR UPDATE statements cause locking rows
for the time of transaction
• Locked objects can be found by following query
SELECT o.owner,
o.object_name,
l.*
FROM v$locked_object l,
dba_objects
o
WHERE l.object_id = o.object_id;
• Sessions waiting for locked resources can be found in
dba_waiters view.
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17
Session view
All the sessions running on Oracle database are visible in v$session
view
TIP use v$session view.
Useful columns of v$session view:
sid,serial#
username
sql_hash_value
action,module
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– uniqe identifier of the session
– name used when connecting the database
– unique identifier of SQL currently executed by the
session
– name of the currently executing action as set by calling
the DBMS_APPLICATION_INFO.SET_ACTION procedure
Session wait view
Displays the current or last wait for each session.
TIP use v$session_wait view.
Useful columns of v$session_wait view:
sid
– uniqe identifier of the session
event
– Resource or event for which the session is waiting
seconds_in_wait – If the session is currently waiting, then the value is
the amount of time waited for the current wait. If the
session is not in a wait, then the value is the
amount
of time since the start of the last wait.
state
– current session state
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List of waiting sessions
Shows all the sessions that are waiting for a lock.
TIP use dba_waiters view.
Useful columns of dba_waiters view:
WAITING_SESSION - The waiting session
HOLDING_SESSION - The holding session
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List of locked objects
Shows all objects currentlu locked
TIP use v$locked_objects view.
Useful columns of v$locked_objects view:
OBJECT_ID
- Locked object ID
SESSION_ID
- The holding session
LOCKED_MODE - Mode of the lock
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List of objects in the database
Shows all objects existing in the database
TIP use dba_objects view.
Useful columns of dba_objects view:
OBJECT_ID
- Uniqye object ID
OWNER
- Owner (schema name)
OBJECT_NAME - Name of the object
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Laboratory database architecture
APP_OWNER
PERSON
FRANIU
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TOY
TOY_HISTORY
BENIU
Practice 1
Both users FRANIU and BENIU are accessing the
PERSON table in APP_OWNER schema.
They try to make update on some rows.
APP_OWNER
Operations from one session was successful
however second session hanged.
PERSON
?Question
•Identify why the second session hang.
•Propose solution.
FRANIU
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BENIU
Practice 2
Both users FRANIU and BENIU are accessing the
PERSON table in APP_OWNER schema.
They try to delete some rows.
APP_OWNER
PERSON
TOY
Operations from one session was successful
however second session hanged.
?Question
•Identify why the second session hang.
•Propose solution.
FRANIU
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BENIU
Practice 3
One session is running operation on the
TOY_HISTORY table.
APP_OWNER
TOY_HISTORY
FRANIU
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?Question
•Identify why the second session hang.
•Propose solution.