Advanced SQL

Advanced SQL

Instructor: Mohamed Eltabakh [email protected]

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Part II

Remaining Lectures

Today Complete Advanced SQL

Dec 6. Brief Introduction to Transactions and

Indexes

Due date for HW4 & Phase 4

Dec 9. Revision + short quiz (from the revision

slides)

Dec 13. Final exam

Today’s Roadmap

Triggers

Assertions

Cursors

Stored Procedures

ODBC/JDBC

Triggers & Assertions

To enforce complex constraints inside the DBMS, we use Triggers or Assertions

Assertions are part of SQL standards, but not all DBMSs support them Triggers are more powerful

Recap on Triggers Three components

Event: When this event happens, the trigger is activated Condition (optional): If the condition is true, the trigger executes,

otherwise skipped Action: The actions performed by the trigger

Create Trigger <name>

Before| After Insert| Update| Delete On <tableName>

Referencing

OLD AS oldRec,

NEW AS newRec

For Each Row | For Each Statement

When <condition>

Begin

….

End;

Event

Event Granularity

Condition

Action

Pointers to old and new records

Example: Maintenance of Derived Attributes

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Keep the bonus attribute in Employee table always 3% of the salary attribute

Create Trigger EmpBonus

Before Insert Or Update On Employee

For Each Row

Begin

newRec.bonus := newRec.salary * 0.03;

End;

The bonus value is always computed automatically

Indicate two events at the same time

Before vs. After Before Event

When checking certain conditions that may cause the operation to be cancelled E.g., if the name is null, do not insert

When modifying values before the operation E.g., if the date is null, put the current date

After Event When taking other actions that will not affect the current operations

The insert in table X will cause an update in table Y

Before Insert Trigger:

newRec.x := …. //Changing value x that will be inserted

After Insert Trigger:

newRec.x := … //meaningless because the value is already inserted

Example

What triggers do we need? After Insert, After Update, After Delete on Employee

For Employees with salary < 50,000, keep the EmpID and salary in table LowSalaryEmp

Create Trigger EmpUpdate

After Update Of salary On Employee

Referencing

OLD AS oldRec, NEW AS newRec

For Each Row

Begin

IF (newRec.sal < 50,000 and oldRec.sal >=50,000) THEN

Insert into LowSalaryEmp values (newRec.EmpId, newRec.sal);

ELSIF (newRec.sal < 50,000 and oldRec.sal < 50,000)

Update LowSalaryEmp set sal = newRec.sal Where empId = newRec.EmpId;

ELSIF (newRec.sal >= 50,000 and oldRec.sal < 50,000)

Delete from LowSalaryEmp Where empId = newRec.EmpId;

END IF;

End;

Can specify which updated column

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Combining Multiple Events in One Trigger

CREATE TRIGGER salaryRestrictionsBEFORE INSERT OR UPDATE ON Professor

Referencing

OLD AS oldRec, NEW AS newRec For Each RowBEGIN

IF (INSERTING AND newRec.salary < 60000) THEN RAISE_APPLICATION_ERROR (-20004, 'below min salary');

END IF;IF (UPDATING AND newRec.salary < oldRec.salary) THEN

RAISE_APPLICATION_ERROR (-20004, ‘Salary Decreasing !!'); END IF;

END;

Can write different code for different events

Summary of Triggers Powerful mechanisms to enforce constraints in the DBMS

Need to know what triggers to create On which tables On which events

Can have many triggers on the same table, possibly of the same type (but different trigger names)

Today’s Roadmap

Triggers

Assertions

Cursors

Stored Procedures

ODBC/JDBC

Assertions

An expression that should be always true

When created, the expression must be true

DBMS checks the assertion after any change that may violate the expression

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Must return True or False

Example 1

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Sum of loans taken by a customer does not exceed 100,000

Create Assertion SumLoans Check

( 100,000 >= ALL

Select Sum(amount)

From borrower B , loan L

Where B.loan_number = L.loan_number

Group By customer_name );

Must return True or False (not a relation)

Example 2

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Number of accounts for each customer in a given branch is at most two

Create Assertion NumAccounts Check

( 2 >= ALL

Select count(*)

From account A , depositor D

Where A.account_number = D.account_number

Group By customer_name, branch_name );

Example 3

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Customer city is always not null

Create Assertion CityCheck Check

( NOT EXISTS (

Select *

From customer

Where customer_city is null));

Example 4 (Exercise)

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The customer city must be as the branch city to have an account or a loan in that branch

Assertions vs. Triggers Assertions do not modify the data, they only check certain

conditions

Triggers are more powerful because the can check conditions and also modify the data

Assertions are not linked to specific tables in the database and not linked to specific events

Triggers are linked to specific tables and specific events

All assertions can be implemented as triggers (one or more)

Not all triggers can be implemented as assertions

Example: Trigger vs. Assertion

All new customers opening an account must have opening balance >= $100. However, once the account is opened their balance can fall below that amount.

We need triggers, assertions cannot be usedTrigger Event: Before Insert

Create Trigger OpeningBal

Before Insert On Customer

Referencing

NEW AS newRec

For Each Row

Begin

IF (newRec.bal is null or newRec.bal < 100) Then RAISE_APPLICATION_ERROR(-20004, ‘Balance should be >= $100’);

End IF;

End;

Today’s Roadmap

Triggers

Assertions

Cursors

Stored Procedures

ODBC/JDBC

What is a Cursor A mechanism to navigate tuple-by-tuple over a relation

Typically used inside triggers, stored procedures, or stored functions

Main Idea When we execute a query, a relation is returned

It is stored in private work area for the query

Cursor is a pointer to this area

Move the cursor to navigate over the tuples

Creating Cursor

Cursor <name> IS <SQL query>;

Cursor HighSalEmp IS

Select empID, name, salary

From Employee

Where salary > 120,000;

Cursor Operations

Create cursor

Open cursor Put pointer to the first tuple

Fetch next tuple Pointer moves automatically when a tuple is fetched

Close cursor

Cursor HighSalEmp IS

Select empID, name, salary

From Employee

Where salary > 120,000;

Open HighSalEmp;

Fetch HighSalEmp into <variable>;

Close HighSalEmp;

ExampleCreate Trigger OpeningBal

After Insert On Customer

Declare

p_Id int; p_name string; p_price number(7,2);

cursor C1 Is //define the cursor

Select productId, name, price

From products

where type = ‘new’;

Begin

open C1; //opened the cursor

Loop

fetch C1 into p_id, p_name, p_price; //fetched the first tuple

IF (C1%Found) THEN // make sure the fetch was successful

….

ELSE

exit; // break the loop

END IF;

End Loop;

close C1; // close the cursor

End;

Another Way



Declare

cursor C1 Is


From products

where type = ‘new’;

Begin

For rec in C1 Loop //opened the cursor

Insert into Temp values (rec.productId, rec.name, rec.price); // fetch values

End Loop; // close the cursor

End;

Cursor Attributes These are attributes maintained by the system

Attributes include: C1%ROWCOUNT: The number of tuples in C1

C1%FOUND: TRUE if the last fetch was successful

C1%NOTFOUND: TRUE if the last fetch was not successful

C1%ISOPEN: TRUE if C1 is open

Parameterized Cursor Cursors can take parameters while opening them



Reference NEW as newRec

Declare

cursor C1(type_Of_Interest string) Is


From products

where type = type_Of_Interest;

Begin

For rec in C1(newRec.type) Loop

Insert into Temp values (rec.productId, rec.name, rec.price);

End Loop;

End;

Define a parameter

Pass the parameter

Summary of Cursors Efficient mechanism to iterate over a relation tuple-by-

tuple

Main operations Open, fetch, close Usually used inside loops

Cursors can be parameterized What they return depends on the passed parameters

Today’s Roadmap

Triggers

Assertions

Cursors

Stored Procedures

ODBC/JDBC

Stored Procedures What is stored procedure?

Piece of code stored inside the DBMS SQL allows you to define procedures and functions and store

them inside DBMS

Advantages Reusability: do not need to write the code again and again Programming language-like environment

Assignment, Loop, For, IF statements Call it whenever needed

From select statement, another procedure or function

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Stored Procedures in Oracle

Stored procedures in Oracle follow a language called PL/SQL

PL/SQL: Procedural Language SQL

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Creating A Stored Procedure

Example: Create Procedure test (id in int, name out string) As Begin

…. End;

CREATE [OR REPLACE] PROCEDURE <procedureName> [(<paramList>)] AS<localDeclarations><procedureBody>;

A parameter in the paramList is specified as:

<name> <mode> <type><mode> is one of {IN, OUT, INOUT}

Example

By default, it is INDefine a variable

Execute the command and create the procedure

In PL/SQL a ‘;’ ends a line without execution

Calling a Stored Procedure SQL> exec <procedureName> [(<paramList>)];

SQL > exec remove_emp (10);

Printing From Stored Procedures

Taking three parameters

Printing them to screen

Features in Stored Procedures

Create Procedure profiler_control(start_stop IN VARCHAR2, run_comm IN VARCHAR2, ret OUT BOOLEAN) AS

ret_code INTEGER;

BEGIN ret_code:=10;

IF ret_code !=0 THEN ret:=FALSE; ELSIF start_stop NOT IN ('START','STOP') THEN ret:=FALSE; ELSIF start_stop = 'START' THEN ret:=FALSE; ELSE ret:=FALSE; END IF;

END profiler_control;/

IN parameters

OUT parameters

Variable declaration

Variable assignment

IF statement

More Features: LOOP Statement

CREATE PROCEDURE testProcedure (name string) ASnum1 int;BEGIN

num1 := 10;LOOP

INSERT INTO Student VALUES (num1, name);

num1 := num1 + 1; IF (num1 > 15) THEN

EXIT; END IF;

END LOOP;END;

More Features: CURSOR & FOR Statement

Create Procedure OpeningBal (p_type IN string) AS

cursor C1 Is


From products

where type = p_type;

Begin

For rec in C1 Loop

Insert into Temp values (rec.productId, rec.name, rec.price);

End Loop;

End;

/

Stored Functions Similar to stored procedures except that they return value

CREATE [OR REPLACE] FUNCTION <functionName> RETURN <type> [(<paramList>)] AS

<localDeclarations><functionBody>;

Using Stored Procedures or Functions

Stored Procedures Called from other procedures, functions, triggers, or

standalone

Stored Functions In addition to above, can be used inside SELECT statement

In WHERE, HAVING, or projection list

Example

CREATE FUNCTION MaxNum() RETURN int AS

num1 int;

BEGIN

SELECT MAX (sNumber) INTO num1 FROM Student;

RETURN num1;

END;

/

SQL> Select * from Student where sNumber = MaxNum();

Summary of Stored Procedures/Functions

Code modules that are stored inside the DBMS

Used and called repeatedly

Powerful programing language style

Can be called from other procedures, functions, triggers, or from select statement (only functions)

Today’s Roadmap

Triggers

Assertions

Cursors

Stored Procedures

ODBC/JDBC

ODBC/JDBC Interfaces that allow applications to connect to a database

and execute queries

Applications can be java, C, C++, C# programs

Application makes calls to Connect with the database server Send SQL commands to the database server Get the results back in your program

ODBC (Open Database Connectivity) works with C, C++, C#, and Visual Basic

JDBC (Java Database Connectivity) works with Java

JDBC JDBC is a Java API for communicating with database systems

supporting SQL

JDBC supports a variety of features for querying and updating data, and for retrieving query results

Model for communicating with the database: Open a connection Create a “statement” object Execute queries using the Statement object to send queries and fetch results Exception mechanism to handle errors

JDBC: Code Example

Connecting to Oracle DB

DB name, port number, userId, password

Holder for SQL statement

JDBC: Code Example (Cont’d)

ODBC

Similar to JDBC, but has its own syntax

Works with C, C++, C# languages

End of Advanced SQL

Triggers

Assertions

Cursors

Stored Procedures

ODBC/JDBC

To check any syntax Google is you friend !!!

Subqueries in DML

Subquery in DML DELETE

DML: Data Manipulation Language

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Subquery in DML UPDATE

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Subquery in DML INSERT

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Follows the order in CREATE TABLE command

Any order of columns

Advanced SQL

Documents

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