64 lines
2.4 KiB
Plaintext
64 lines
2.4 KiB
Plaintext
== What is SQL
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SQL is a standardized (ANSI in 1986, ISO in 1987) programming language which is used for managing relational databases and performing various operations on the data in them.
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A database is a collection of data. Data is organized into rows, columns and tables, and it is indexed to make it easier to find relevant information.
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Example SQL table with employees:
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Employees Table
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|===
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|IdNum |LName |FName |JobCode |Salary |Phone |
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|1876 |CHIN |JACK |TA1 |42400 |212/558-5634 |
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|1114 |GREENWALD |JANICE |ME3 |38000 |212/558-1092 |
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|1556 |PENNINGTION|MICHAEL |ME1 |29860 |718/383-5681 |
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|1354 |PARKER |MARY |FA3 |65800 |914/455-2337 |
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|1130 |WOOD |DEBORAH |PT2 |36514 |212/587-0013 |
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|===
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Each employee has an index (IdNum), lastname, firstname, job title (JobCode), salary and a phone number. All his data from this table is represented in a single row.
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By using SQL queries you can modify a database table and its index structures, add, update and delete rows of data.
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There are three types of SQL commands in the SQL database language:
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=== Data Manipulation Language (DML)
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* DML commands are used for storing, retrieving, modifying, and deleting data.
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* SELECT, INSERT, UPDATE, DELETE, …
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* Example:
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** Retrieve data:
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** SELECT Phone +
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FROM Employees +
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WHERE IdNum = 1354;
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** This statement delivers the phone number of the employee with the number 1354.
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=== Data Definition Language (DDL)
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* DDL commands are used for creating, modifying, and dropping the structure of database objects.
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* CREATE, ALTER, DROP,TRUNCATE,…
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* Example:
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** CREATE TABLE Customers( +
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IdNum INT NOT NULL, +
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LName VARCHAR (20) NOT NULL, +
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FName VARCHAR (20) NOT NULL, +
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JobCode VARCHAR (3) NOT NULL, +
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Salary DECIMAL (18, 2), +
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Phone VARCHAR (20), +
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PRIMARY KEY (IdNum) +
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);
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** This statement creates the employees example table given above.
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=== Data Control Language (DCL)
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* DCL commands are used for providing security to database objects.
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* GRANT, REVOKE, …
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* Example:
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** GRANT CREATE TABLE +
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TO operator;
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** This statement gives all users of the operator-role the privilege to create new tables in the database.
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If you are still struggling with SQL and need more information or practice you can visit http://www.sqlcourse.com/ for an interactive and free online training.
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