Structured Query Language (1) (SQL) Eng. Mohammed Alokshiya

Islamic University of Gaza Faculty of Engineering Computer Engineering Dept. Database Lab (ECOM 4113)

Lab 2

Structured Query Language (1) (SQL)

Eng. Mohammed Alokshiya

October 14, 2014

SQL is a standard language for accessing and manipulating databases. SQL commands are instructions, coded into SQL statements, which are used to communicate with the database to perform specific tasks, work, functions and queries with data. SQL commands can be used not only for searching the database but also to perform various other functions like, for example, you can create tables, add data to tables, or modify data, drop the table, set permissions for users. SQL commands are grouped into major categories depending on their functionality:  Data Manipulation Language (DML) - These SQL commands are used for storing, retrieving, modifying, and deleting data. These Data Manipulation Language commands are CALL, DELETE, EXPLAIN, INSERT, LOCK TABLE, MERGE, SELECT and UPDATE.  Data Definition Language (DDL) - These SQL commands are used for creating, modifying, and dropping the structure of database objects. The commands are ALTER, ANALYZE, AUDIT, COMMENT, CREATE, DROP, FLASHBACK, GRANT, PURGE, RENAME, REVOKE and TRUNCATE.  Transaction Control Language (TCL) - These SQL commands are used for managing changes affecting the data. These commands are COMMIT, ROLLBACK, and SAVEPOINT.  Etc.

In this lab, we will use the well-known HR sample schema (which included in Oracle XE).

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INSERT Statement

INSERT statement is used to add new rows into a table in the database. The syntax of SQL INSERT statement is: Syntax of SQL INSERT Statement INSERT INTO table-name (column_list) VALUES (values-list);

Example: Using INSERT Statement INSERT INTO COUNTRIES (COUNTRY_ID, COUNTRY_NAME, REGION_ID) VALUES ('PS', 'Palestine', 4);

You can also insert new rows without specifying column names, by typing: “INSERT INTO table-name VALUES (value-list)”. In this case, you MUST order values in the same order of its corresponding columns. Example: Using INSERT Statement INSERT INTO COUNTRIES VALUES ('PS', 'Palestine', 4);

Note: use “DESC[RIBE] table-name” statement to see the columns order in table definition.

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DELETE Statement

DELETE statement is the opposite of INSERT statement. It is used to delete rows from a table according to a specified condition. The syntax of SQL DELETE statement is: Syntax of SQL DELETE Statement DELETE FROM table-name WHERE condition;

Example: Using DELETE Statement DELETE FROM COUNTRIES WHERE COUNTRY_ID = 'IL';

It also can be used to delete all rows from a table by not specifying any conditions. If you want to empty a table, you just have to issue this command: “DELETE FROM table-name”. You should be aware when using this form. UPDATE Statement

UPDATE statement is used to modify existing rows values in a table. The syntax of SQL UPDATE statement is: Syntax of SQL UPDATE Statement UPDATE table-name SET column_name_1 = new_value1 , column_name_2 = new_value2 -- etc WHERE condition;

Example: Using UPDATE Statement UPDATE JOBS SET MIN_SALARY = 30000, MAX_SALARY = 50000 WHERE JOB_TITLE = 'President';

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SELECT Statement

The most commonly used SQL command is SELECT statement. SQL SELECT statement is used to query or retrieve data from a table in the database. A query may retrieve information from specified columns or from all of the columns in the table. To create a simple SQL SELECT Statement, you must specify the column(s) name and the table name. The whole query is called SQL SELECT Statement. Syntax of SQL SELECT Statement SELECT * | {[DISTINCT] column_list | expression [alias],...} FROM table-name [WHERE condition] [GROUP BY columns] [HAVING group-selection-condition] [ORDER BY column-names || aliases || column-numbers]; - table-name is the name of the table from which the information is retrieved. - column_list includes one or more columns from which data is retrieved.

 Selecting All Columns Selecting All Columns SELECT * FROM EMPLOYEES;

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Press F9 to run the statement:

 Selecting Specific Columns Selecting Specific Columns SELECT FIRST_NAME, LAST_NAME, PHONE_NUMBER, SALARY FROM EMPLOYEES;

 Arithmetic Expressions Create expressions with number and date data by using arithmetic operators Using Arithmetic Operators SELECT FIRST_NAME, LAST_NAME, SALARY, SALARY + 300 FROM EMPLOYEES;

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 Null Values  A null is a value that is unavailable, unassigned, unknown, or inapplicable.  It is NOT the same as a zero or a blank space. Null Values SELECT LAST_NAME, JOB_ID, SALARY, COMMISSION_PCT FROM EMPLOYEES;

 You cannot use equal operator (=) to compare two null values! Instead, use (IS) key word to check if a value is null or not. Example: retrieve all employees who have not manager Null Values SELECT FIRST_NAME, LAST_NAME, MANAGER_ID FROM EMPLOYEES WHERE MANAGER_ID IS NULL;

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The result of any arithmetic expressions containing a null value is a null value o null + 5 is null o null * 12 is null Null Values in Arithmetic Expressions SELECT LAST_NAME, SALARY,COMMISSION_PCT, SALARY + SALARY * COMMISSION_PCT FROM EMPLOYEES;

o To overcome this problem, use NVL function, which converts a Null value into an actual specified value Using NVL Function SELECT LAST_NAME, SALARY, COMMISSION_PCT, SALARY + SALARY * NVL(COMMISSION_PCT, 0) FROM EMPLOYEES;

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 A Column Alias  Renames a column heading  Is useful with calculations  Immediately follows the column name (There can also be the optional AS keyword between the column name and alias.)  Requires double quotation marks if it contains spaces or special characters or if it is case-sensitive Using Aliases SELECT FIRST_NAME FName, LAST_NAME AS LName, SALARY * 12 "Annual Salary" FROM EMPLOYEES;

 A Concatenation Operator  Links columns or character strings to other columns  Is represented by two vertical bars (||)  Creates a resultant column that is a character expression

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Concatenation Operator SELECT FIRST_NAME || ' ' || LAST_NAME AS "Full Name" FROM EMPLOYEES;

 Duplicate Rows  The default display of queries is all rows, including duplicate rows, but you can display only distinct values without duplication by using “DISTINCT” keyword. SELECT DISTINCT SELECT DISTINCT DEPARTMENT_ID FROM EMPLOYEES;

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Restricting Data

In most cases, we query the database to get some specific data from a table and not the whole table; therefore, we need a technique to restrict the data retrieved by SELECT statement. Restricting the rows that are returned by a query can be done by using the optional [WHERE clause]. Syntax of SQL SELECT Statement SELECT * | {[DISTINCT] column_list | expression [alias],...} FROM table-name [WHERE condition]

Example: retrieving all employees who are working in depart 30. All Employees in Deptartment 30 SELECT * FROM EMPLOYEES WHERE DEPARTMENT_ID = 30;

 Notes:  When you deal with character strings or date values, you must enclosed them by single quotation marks (' ').  Character values are case-sensitive, and date values are format-sensitive.  The default date format is DD-MON-RR.

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Example: retrieving all employees whose first name is “David”. All David’s SELECT * FROM EMPLOYEES WHERE FIRST_NAME = 'David';

Example: retrieving all employees who are hired on 7/6/2002. All Employees with Hire Date: 7/6/2002 SELECT * FROM EMPLOYEES WHERE HIRE_DATE = '7-JUN-02';

 LIKE Condition  Use the LIKE condition to perform wildcard searches of valid search string values.  Search conditions can contain either literal characters or numbers: o % denotes zero or many characters. o _ denotes one character Example: retrieve all employees whose first name starts with “A” SELECT FIRST_NAME, LAST_NAME FROM EMPLOYEES WHERE FIRST_NAME LIKE 'A%';

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Example: retrieve all employees whose first name starts with “A” and the third letter is “e” SELECT FIRST_NAME, LAST_NAME FROM EMPLOYEES WHERE FIRST_NAME LIKE 'A_e%';

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Comparison operators:

Operator = > >= < , =, = 5000 AND SALARY ALL(9000, 4800, 4200, 6000);

And you will get the targeted employees: The previous solution is not wrong. However, it is not a practical solution since you apply it with many steps and each step needs a human to do them (cannot applied by machine since the retrieve values are dynamic and can be changed in any time. A better solution is to embedded the first query in the second query in “All’s condition parentheses”, which is called: inner query: All Employees in Departments: 60, 90, and 100 SELECT FIRST_NAME, LAST_NAME, SALARY FROM EMPLOYEES WHERE DEPARTMENT_ID = 100 AND SALARY > ALL( SELECT DISTINCT SALARY FROM EMPLOYEES WHERE DEPARTMENT_ID = 60 );

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Sorting Data

You can sort rows retrieved by a query using the optional [ORDER BY clause]. Syntax of SQL SELECT Statement SELECT * | {[DISTINCT] column_list | expression [alias],...} FROM table-name [WHERE condition] [ORDER BY column-names || expression || aliases || (column-alias)number];

Example: retrieving all employees’ records sorted by first name. Using “Order By” Clause SELECT FIRST_NAME, LAST_NAME, HIRE_DATE, SALARY FROM EMPLOYEES ORDER BY FIRST_NAME;

By default, “ORDER BY” Clause sorts the retrieved rows in ascending order. To reverse the ordering, use “DESC” keyword after column-name. Example: retrieve all employees’ records sorted by their hire date. However, the newest employee should come first and the old one come last. Descending Order SELECT FIRST_NAME, LAST_NAME, HIRE_DATE, SALARY FROM EMPLOYEES ORDER BY HIRE_DATE DESC;

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You can also sort rows according to expression result; in this case, you have to use an alias instead of column name. Example: retrieve all employees sorted by their total salary. Total salary = salary + salary * commission_pct Descending Order SELECT FIRST_NAME, LAST_NAME, SALARY + SALARY * NVL(COMMISSION_PCT, 0) "Total Salary" FROM EMPLOYEES ORDER BY "Total Salary" DESC;

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You can sort rows according to more than one column. Example: retrieve all employees sorted by their total salary, if there are two employees have the same total salary, then sort them by first name in ascending order, then by their last name. Sorting According to More Than One Column SELECT FIRST_NAME, LAST_NAME, SALARY + SALARY * NVL(COMMISSION_PCT, 0) "Total Salary" FROM EMPLOYEES ORDER BY "Total Salary" DESC, FIRST_NAME, LAST_NAME;

Note: in “ORDER BY” clause, you can type (column | alias) numeric position instead of name. For example in the previous query, FIRST_NAME column comes first in the query, so its number is 1. LAST_NAME comes second, so its number is 2. Finally, “Total Salary” alias comes third, so its number is 3. Therefore, the previous query can be wrote in another way: Sorting According to More Than One Column SELECT FIRST_NAME, LAST_NAME, SALARY + SALARY * NVL(COMMISSION_PCT, 0) "Total Salary" FROM EMPLOYEES ORDER BY 3 DESC, 1, 2;

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Exercises: 1. Write SQL statement to insert a new record into JOBS table with

the following information: JOB_ID = C_ENG JOB_TITLE = Computer Engineer MIN_SALARY = 20000 MAX_SALARY = 50000

2. Write SQL statement to modify all employees’ JOB_IDs to

C_ENG, if there salary is between 20000 and 50000. 3. Write SQL statement to delete all employees who are not

working in any departments, and all employees who are working in department 80, but their TOTAL salary is less than 7000.  Note: total salary = salary + salary * commission_pct 4. Write SQL statement to retrieve the last name and salary for all

employees whose salary is not in the range 5000 through 12000. 5. Write SQL statement to retrieve the last names of all employees

who have both an “a” and an “e” in their last name. 6. Write SQL statement to retrieve the last name, salary, and

commission for all employees who earn commissions. Sort data in descending order of salary and commissions. Use the column’s numeric position in the ORDER BY clause. 7. Write SQL statement to retrieve all employee last names in

which the third letter of the name is “a”.

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