Block 3 The database language SQL. Block 3 The database language SQL. SQL Control Statements. SQL Control Statements. There are seven sections.

Size: px

Start display at page:

Download "Block 3 The database language SQL. Block 3 The database language SQL. SQL Control Statements. SQL Control Statements. There are seven sections."

Gerald Parks
5 years ago
Views:

Block 3 The database language SQL Block 3 The database language SQL There are seven sections. Section 1. Introduction Section 2. Retrieval Using Simple Queries Section 3.

Defining Views 2. Sharing Data Section 7. SQL control statements OUHK S359 Relational Databases: theory and practice T7-1 OUHK S359 Relational Databases: theory and practice T7-2 1.

1 Block 3 The database language SQL Block 3 The database language SQL There are seven sections. Section 1. Introduction Section 2. Retrieval Using Simple Queries Section 3. Retrieval Using Composite Queries Section 4. Use of NULL Section 5. Database Management and Administration.. There are seven sections. Section 6. Database management and administration 1. Defining Views 2. Sharing Data Section 7. SQL control statements OUHK S359 Relational Databases: theory and practice T7-1 OUHK S359 Relational Databases: theory and practice T SQL Routines It is possible to create own SQL routines. There are two types. One is function and another is procedure. Some differences in the way these facilities are supported by different DBMSs. 1. SQL Routines Compound statements: a number of statements can be defined as a single statement using the keyboards BEGIN and END. Has the same effect as executing two or more SQL statements separately. DBMS treats compound statement as a single action Can also be defined as ATOMIC. An ATOMIC compound statement must either succeed completely or fail completely. OUHK S359 Relational Databases: theory and practice T7-3 OUHK S359 Relational Databases: theory and practice T7-4

2 1. SQLRoutines Function creation Create Function Check_fee (sid integer, cid integer) Rt Returns integer it Begin Declare c_fee decimal (5,2); Select course_fee into c_fee from enrolment where student_ id = sid and course_ id = cid; returns c_fee; End OUHK S359 Relational Databases: theory and practice T SQL Routines Function execution Select s.student_name, student name Check_fee (s.student_id, student s.course_id) as fee from student t where group_no = 3 ; OUHK S359 Relational Databases: theory and practice T SQL Routines - Procedure Also known as stored procedure or database procedures. Similar to Function in that they are routines with a bd body containing tii a compound statement. tt t Unlike Function, procedure does not return a value and so does not have a RETURNS clause or a RETURN statement in the body. can be used to put values into one or more variables. ibl OUHK S359 Relational Databases: theory and practice T SQL Routines Procedure creation Create Procedure Pid_Weight_Charge (IN pid integer, OUT weight integer, OUT charge integer) Begin Select patient_weight, patient_charge into weight, charge from patient where patient_id = pid; End OUHK S359 Relational Databases: theory and practice T7-8

3 1. SQL Routines Procedure execution Begin declare pat_weight decimal (4,2); declare pat_charge decimal (4,2); CALL Pid_Weight_Charge i (pat_id, pat_weight, pat_charge) End 1. SQL Routines Deleting routines Standard SQL: Same Function / Procedure name is allowed Drop Function <function_name> [<data_type>, ] Drop Procedure <procedure_name> [<data_type>, ] SQLAnywhere: Same Function / Procedure name is not allowed Drop Function <function_name> Drop Procedure <procedure_name> OUHK S359 Relational Databases: theory and practice T7-9 OUHK S359 Relational Databases: theory and practice T SQL Routines Function Vs Procedure Function can be included in an expression for any SQL Procedure can be invoked by CALL statement Function is more flexible Function can return only a single result Procedure can return many values Procedure is better. 1. SQL Routines Benefits (a) Location of Functionality A single routine can be kept by the database rather than requiring each of the different application programs to define their own routines Routines can be used to hide unnecessary complexity (e.g. database structure) from database users. If database is restructured, t all applications using the procedure do not need to be updated if the procedure is redefined. OUHK S359 Relational Databases: theory and practice T7-11 OUHK S359 Relational Databases: theory and practice T7-12

4 1. SQL Routines Benefits (b) Security Routines may also be used to provide restricted access to the underlying data in a database to users who do not have the right to modify or access that data directory. By ygranting gexecute privilege on a procedure/function 1. SQLRoutines Benefits (c) () Efficiency Routines that are defined within the DBMS can access the data that they require without needing to communicate, as they execute on the DBMS server. When a single routine is required by many application programs, the concurrency in the execution can be handled by the DBMS, rather than relying upon access by several application programs. OUHK S359 Relational Databases: theory and practice T7-13 OUHK S359 Relational Databases: theory and practice T Cursors for row-by-row processing 2. Cursors for row-by-row processing CALL high_mark(50) Result: The table exam_mod contains student identifier, course code and dfinal mark of every student tin the examination table with an examination mark of 50 or higher. OUHK S359 Relational Databases: theory and practice T7-15 OUHK S359 Relational Databases: theory and practice T7-16

5 2. Cursors for row-by-row processing - Highlights SQLSTATE variable One special global variable whose value is set by DBMS following the execution of every SQL statement to indicate the status of the processing of the statement. tt t 2. Cursors for row-by-row processing - Highlights Control Flow Statements: WHILE <condition> LOOP < compound statement> END LOOP OUHK S359 Relational Databases: theory and practice T7-17 OUHK S359 Relational Databases: theory and practice T Cursors for row-by-row processing - Highlights Control Flow Statements: 2. Cursors for row-by-row processing - Highlights Update using cursors: - positioned updates No explicit OPEN statement; No explicit FETCH statement; No explicit condition for terminating the loop. OUHK S359 Relational Databases: theory and practice T7-19 The cursor is placed so that the subsequent FETCH statement retrieves the next row. The positioned update does not affect the position of the cursor. OUHK S359 Relational Databases: theory and practice T7-20

6 3. Triggers - A type of stored procedure that uses a compound statement but does not normally return a result. - A trigger is associated with a single named table, known as its subject table, such that when the table is modified by an INSERT, DELETE or UPDATE event, the triggers associated with the particular operation on that tables are fired, and compound statement defined within the trigger is executed. 3. Triggers - The essential difference between routines and triggers is that a routine is explicitly invoked by a user a trigger is implicitly invoked as a consequence of some update of its subject table, without the user who caused the update being aware of the triggered action. OUHK S359 Relational Databases: theory and practice T7-21 OUHK S359 Relational Databases: theory and practice T Triggers - example CREATE TRIGGER add_enrolment AFTER INSERT ON mod_enrolment REFERENCING NEW AS new_enrolment FOR EACH ROW BEGIN ATOMIC UPDATE mod_course SET student_count = student_count + 1 WHERE mod_course.course_code = new_enrolment.course_code; END OUHK S359 Relational Databases: theory and practice T Ti Triggers -example AFTER INSERT - states that the trigger is fired when an INSERT event is carried out. Other possible events are DELETE or UPDATE. Triggers can be specified as BEFORE. REFERENCING NEW AS new_ enrolment - States that the new row of data can be referred to as new_enrolment within the body of the trigger action. - OLD is used for DELETE; - OLD or NEW is used for UPDATE. OUHK S359 Relational Databases: theory and practice T7-24

7 3. Ti Triggers -example 3. Ti Triggers - REMOVING Triggers - format: DROP TRIGGER <trigger name> - example: DROP TRIGGER add_enrolment OUHK S359 Relational Databases: theory and practice T7-25 OUHK S359 Relational Databases: theory and practice T Transaction Management 4. Transaction Management - DBMS should be able to resume the balance back to original image when there is any failure before the completion of transaction. Transaction Recovery - DBMS should also be able to ensure that a transaction completes its processing without the data involved being affected by other programs. Concurrency Control OUHK S359 Relational Databases: theory and practice T7-27 OUHK S359 Relational Databases: theory and practice T7-28

8 4. Transaction Management - Transaction Recovery - If a transaction succeeds, the program requests the DBMS to commit all the changes made during the course of the transaction to the database. - If a transaction fails, the program requests the DBMS to rollback all the changes made during the course of the transaction so that the database returns to its state at the start of the transaction. 4. Transaction Management - Transaction Recovery - SQL statements: Commit work / Rollback work ACommit work statement causes all statements in a transaction to be recorded in a database. A Rollback work statement ensures that a transaction being completed has no effect on a database OUHK S359 Relational Databases: theory and practice T7-29 OUHK S359 Relational Databases: theory and practice T Transaction Management - Transaction Recovery - A new transaction is considered to have started automatically either at the beginning of a program or where the previous transaction ended. - For example, you have typed the following SQL statements at SQL promptp SQL > insert into student (student_id, student_name) value ( 00011, Peter ); SQL > update student set student_mark = 90 where student_id in ( 00001, 00002, ); SQL > OUHK S359 Relational Databases: theory and practice T Transaction Management - Transaction Recovery - At this moment, if you issue SQL statement commit work, then all Insert and update statements are commited. i d - On the contrary, if you issue SQL statement rollback work, then all Insert and update statements are rollback and the images of all records are the same as before. OUHK S359 Relational Databases: theory and practice T7-32

9 4. Transaction Management - Transaction Recovery - Savepoint: a marker that can be put in in the transaction so that when a ROLLBACK statement is used, all the work up to that savepoint is kept, and only the work that comes after the savepoint is rolled back. 4. Transaction Management - Transaction Recovery OUHK S359 Relational Databases: theory and practice T7-33 OUHK S359 Relational Databases: theory and practice T Transaction Management 4. Transaction Management Lost update problem Problems that can occur with Concurrent Transaction 1. Lost update problem 2. Uncommitted data (or dirty read) problem 3. Non-repeatable read problem 4. Phantom rows problem OUHK S359 Relational Databases: theory and practice T7-35 OUHK S359 Relational Databases: theory and practice T7-36

10 4. Transaction Management - Lost update problem 4. Transaction Management Dirty read problem OUHK S359 Relational Databases: theory and practice T7-37 OUHK S359 Relational Databases: theory and practice T Transaction Management Non-repeatable tbl read problem 4. Transaction Management Phantom row problem OUHK S359 Relational Databases: theory and practice T7-39 OUHK S359 Relational Databases: theory and practice T7-40

11 4. Transaction Management Serialized execution - To solve the above problems, a simplistic way for a DBMS to allow only one user to execute a transaction at a time. Only when one user s transaction has finished may another user s transaction ti start. t This is known as serialized execution. - If DBMS can provide isolation equivalent to serialized execution while executing the transactions concurrently, we say that the execution of concurrent transactions is serializable. 4. Transaction Management Serializable execution - This requires DBMS to ensure that the result is the same as some serialized execution of transactions. - To implement a serializable execution, locking is required. - ADBMS processes a concurrent ttransaction ti by marking any data involved in the transaction as locked. If another transaction needs access to data that is locked, it is forced to wait until the locking transaction is completed and the lock on the data is released. OUHK S359 Relational Databases: theory and practice T7-41 OUHK S359 Relational Databases: theory and practice T Transaction Management level lof concurrent access 4. Transaction Management level lof concurrent access - The level of concurrent access for a transaction is determined by its isolation level. - SQL > set transaction isolation level <level> - The isolation level can be set for each transaction OUHK S359 Relational Databases: theory and practice T7-43 OUHK S359 Relational Databases: theory and practice T7-44

12 4. Transaction Management level lof concurrent access - The highest level is known as serializable which guarantees serializable execution - The lowest level el may result in one of the concurrency problems. ~ END ~ - The choice is a trade-off between accuracy of data and speed of execution. OUHK S359 Relational Databases: theory and practice T7-45 OUHK S359 Relational Databases: theory and practice T7-46

Weak Levels of Consistency

Weak Levels of Consistency - Some applications are willing to live with weak levels of consistency, allowing schedules that are not serialisable E.g. a read-only transaction that wants to get an approximate