15 Structured Query Language (SQL)

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ObjectivesObjectives

After completing this section, you should be able to:• Understand Structured Query Language (SQL) and its

purpose• Create and execute a basic SELECT statement• Create and execute a basic INSERT statement• Create and execute a basic UPDATE statement• Create and execute a basic DELETE statement• Execute COMMIT and ROLLBACK on transaction

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What is SQL?What is SQL?

SQL (Structured Query Language) is a standard interactive programming language for getting information from and updating to a database.

Statements take the form of a command language that lets you:

• SELECT data

• INSERT data

• UPDATE data

• DELETE data

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Guidelines in writing SQL StatementsGuidelines in writing SQL Statements

• SQL statements are not case-sensitive• SQL statements can be on one or more lines• SQL statements are optionally ended with “;”• Keywords cannot be abbreviated or split across lines• Clauses are usually placed on separate lines• Indents are used to enhance readability• Keywords are typically entered in uppercase; all other

words such as table name and columns are entered in lower case

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Basic SELECT StatementBasic SELECT StatementUse the SELECT statement to retrieve data from one or more tables:

SELECT <column(s) >

FROM <table>

[WHERE <condition>]

[ORDER BY <column(s) [ASC|DESC]>]

table is the name of the table

column is the name of the column in the table to be selected

condition identifies the rows to be selected and is composed of column names, expressions, constraints, sub-queries and comparison operators

column is the name of the column(s) used for sorting(order by)

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Choosing ColumnsChoosing Columns

• To choose all the columns of the table for display, you can use the asterisk (*)

SELECT *

FROM TABLE_A

• To choose specific columns on the table for display, you specify each column separated by a comma (,)

SELECT COLUMN_1,

COLUMN_2

FROM TABLE_A

• It is best to put each column chosen in a separate line

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Limiting RowsLimiting Rows

• The method of restriction is the basis of the WHERE clause in SQL

• Character strings and dates in the WHERE clause must be enclosed in Single Quotation Marks (‘)

• Numeric Values do not need the Single Quotation marks(‘)

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Rows may be limited by:Rows may be limited by:

• EQUALS CONDITION• Display rows based on an exact match of values.

SELECT last_name, salary

FROM employee WHERE salary = 30000

SELECT employee_id, last_name

FROM employee WHERE manager_name = ‘RAYMOND’

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Rows may be limited by:Rows may be limited by:

• >, < or <> CONDITION

SELECT last_nameFROM employee WHERE salary > 30000

SELECT employee_idFROM employee WHERE manager_name <= ‘RAYMOND’

SELECT employee_idFROM employee WHERE status <> ‘ACTIVE’

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Rows may be limited by:Rows may be limited by:

• BETWEEN CONDITION• Display rows based on a range of values

SELECT last_nameFROM employee WHERE salary BETWEEN 30000 AND 50000

• IN CONDITION• Display rows based on a list of values

SELECT employee_idFROM employee WHERE manager_id IN (100, 200, 300)

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Rows may be limited by:Rows may be limited by:

• LIKE CONDITION• Perform wildcard searches of valid search string

values• Can contain either literal characters or numbers

• % denotes zero or many characters• _ denotes one character

• Use ESCAPE identifier to search for the actual % and _symbols. Identifies the backslash(\) as the escape character

SELECT last_name FROM employee WHERE last_name LIKE ‘%a’

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Rows may be limited by:Rows may be limited by:

• LOGICAL CONDITION• AND, OR, NOT

SELECT last_name, job_id

FROM employee WHERE job_id NOT IN ('SSE', 'TL')

SELECT last_name, job_id

FROM employee WHERE salary NOT between 10000 AND 15000

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Rows may be limited by:Rows may be limited by:

• LOGICAL CONDITION• AND, OR, NOT

SELECT last_name,job_id

FROM employee WHERE last_name NOT LIKE 'A%'

AND last_name NOT LIKE 'B%‘

SELECT last_name,job_id

FROM employee WHERE commission_pct IS NOT NULL

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Sorting RowsSorting Rows

• ORDER BY clause• ASC specifies an ascending order• DESC specifies a descending order

SELECT last_name, salary, job_id

FROM employee ORDER BY salary DESC, job_id ASC

• Display the result in descending order by the attribute salary. If two records have the same attribute value, the salary sorting criteria is in ascending order according to the attribute values of job_id

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Basic INSERT StatementBasic INSERT Statement

INSERT INTO <table>

[ (column

[, column…] ) ]

VALUES (value

[, value…] )

table is the name of the tablecolumn is the name of the column in the table to

populatevalue is the corresponding value for the column

Note: This statement with the VALUES clause adds only one row at a time to a table.

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Basic INSERT StatementBasic INSERT Statement

• Insert a new row containing values for each column• List values in the default order of the columns in the table• Option: list the columns in the INSERT clause• Enclose character and date values within single quotation marks

INSERT INTO departments ( department_id, department_name, current_date)

VALUES ( 70, ‘Public Relations’,’10-OCT-04’)

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Inserting Rows from Another TableInserting Rows from Another Table

• Write your INSERT statement with a subquery• Do not use the VALUES clause• Match the number of columns in the INSERT clause to

those in the subquery

INSERT INTO sales_reps ( id,name,salary)

SELECT employee_id,last_name,salary

FROM employeesWHERE job_id LIKE ‘%REP%’

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Basic UPDATE StatementBasic UPDATE Statement

UPDATEtable

SET column = value

[, column = value, …]

[WHERE condition]

table is the name of the table

column name of the column in the table to populate

value corresponding value for the column

condition identifies the rows to be updated and is composed of column names, expressions, constraints, sub-queries, and comparisonoperators

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Updating Rows in a TableUpdating Rows in a Table

• Specific row or rows are modified if you specify the WHERE clause

• All rows in the table are modified if you omit the WHERE clause

UPDATE employeesSET department_id = 70WHERE employee_id = 113

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Updating Rows Based on Another TableUpdating Rows Based on Another Table

• Use subqueries in UPDATE statements to update rows in a table based on values from another table

UPDATE copy_empSET department_id = (SELECT department_id

FROM employees WHERE emp_id =100)

WHERE job_id = (SELECT job_id FROM employees WHERE emp_id = 200)

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Basic DELETE StatementBasic DELETE Statement

DELETE [FROM] table

[WHEREcondition] ;

table is the name of the table

condition identifies the rows to be updated and is composed of column names, expressions, constraints, sub-queries, and comparisonoperators

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Deleting Rows in a TableDeleting Rows in a Table

• A specific row or specific rows are deleted if you specify the WHERE clause

• All rows in the table are deleted if you omit the WHERE clause

DELETE FROM employeesWHERE employee_id = 113 ;

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Deleting Rows Based on Another TableDeleting Rows Based on Another Table

• Use subqueries in DELETE statements to delete rows in a table based on values from another table

DELETE FROM employeesWHERE department_id = (SELECT department_id

FROM departments WHERE dept_type = ‘CST’)

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What is a Transaction?What is a Transaction?

• A transaction usually means a sequence of information exchange and related work (such as database updating) that is treated as a unit for the purposes of satisfying a request and for ensuring database integrity

• For a transaction to be completed and database changes to be made permanent, a transaction has to be completed in its entirety. A program that manages or oversees the sequence of events that are part of a transaction is sometimes called a transaction monitor

• Transactions are supported by SQL. When a transaction completes successfully, database changes are said to be committed; when a transaction does not complete, changes are rolled back

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Transaction ControlTransaction Control

• Transaction Control Statements• COMMIT • ROLLBACK

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COMMITCOMMIT

• Use the COMMIT statement to end your current transaction and make permanent all changes performed in the transaction

• If you do not explicitly commit the transaction and the program terminates abnormally, then the last uncommitted transaction is automatically rolled back

• SYNTAX:

COMMIT;

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ROLLBACKROLLBACK

• Use the ROLLBACK statement to undo work done in the current transaction, or to manually undo the work done by an in-doubt distributed transaction

• SYNTAX:

ROLLBACK;

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Key PointsKey Points

• SQL is an industry standard language for updating, and getting information from, a database

• The basic and most common SQL statements are: SELECT, INSERT, UPDATE, DELETE

• Transaction management is implemented in SQL using COMMIT and ROLLBACK