overview of sql basics part 1 - by software outsourcing company india

Prepared By:Mr. Dhrumil Patel

iFour Consultancy

Basics of SQL

Introduction to SQL

What is SQL?

SQL is a standard language for accessing databases. SQL stands for Structured Query Language. SQL lets you access and manipulate databases. Nonprocedural language. SQL is an ANSI (American National Standards Institute) standard.

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Database

A database is an organized collection of data, typically stored in electronic format:It allows you to input, manage, organize, and retrieve data quicklyTraditional databases are organized by records (rows), fields (columns) stored in tables

which are stored in the database filesDatabases support storage and manipulation of data. Databases make data management easy.

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Table in SQL

A database table is a collection of rows and columns that is used to organize information about a single topic. Each row within a table corresponds to a single record and contains several attributes that describe the row.

These tables are stored in databases.




SQL

Data Definition Language (DDL)SQL includes commands to:

Create database objects, such as tables, indexes, and viewsDefine access rights to those database objects

Data Manipulation Language (DML)Includes commands to insert, update, delete, and retrieve data within database tables




SQL (Cont.)

SQL Data Definition Commands:

COMMAND OR OPTION DESCRIPTIONCREATE SCHEMA AUTHORIZATION CREATE TABLE NOT NULL UNIQUE PRIMARY KEY FOREIGN KEY DEFAULT CHECK CREATE INDEX CREATE VIEW ALTER TABLE CREATE TABLE AS DROP TABLE DROP INDEX DROP VIEW

Creates a database schema.Creates a new table in the user’s database schema.Ensures that a column will not have null values.Ensures that a column will not have duplicate values.Defines a primary key for a table.Defines a foreign key for a table.Defines a default value for a column (when no value is given).Constraint used to validate data in an attribute.Creates an index for a table.Creates a dynamic subset of rows/columns from one or more tables.Modifies a table's definition (adds, modifies, or deletes attributes or constraints).Creates a new table based on a query in the user's database schema.Permanently deletes a table (and thus its data).Permanently deletes an index.Permanently deletes a view.




SQL (Cont.) SQL Data Manipulation Commands:

COMMAND OR OPERATION DESCRIPTION

INSERT Inserts row(s) into a table.

SELECTWHEREGROUP BYHAVINGORDER BY

Selects attributes from rows in one or more tables or views.Restricts the selection of rows based on a conditional expression.Groups the selected rows based on one or more attributes.Restricts the selection of grouped rows based on a condition.Orders the selected rows based on one or more table’s rows.

UPDATE Modifies an attribute’s values in one or more table’s rows.

DELETE Deleted one or more rows from a table.

COMMIT Permanently saves data changes.

ROLLBACK Restores data to their original values.




SQL (Cont.)

COMPARISON OPERATION =,<,>,<=,>=,<> Used in conditional expressions.

LOGICAL OPERATION AND/OR/NOT Used in conditional expressions.

SPECIAL OPERATION BETWEEN IS NULL LIKE IN EXISTS DISTINCT

Used in conditional expressions.Checks whether an attribute value is within a range.Checks whether an attribute value is null.Checks whether an attribute value matches a given string pattern.Checks whether an attribute value matches any value within a value list.Checks whether a subquery returns any rows.Limits values to unique values.

Aggregate Functions COUNT MIN MAX SUM AVG

Used with SELECT to return mathematical summaries on columns.Returns the number of rows with non-null values for a given column.Returns the minimum attribute value found in a given column.Returns the maximum attribute value found in a given column.Returns the sum of all values for a given column.Returns the average of all values for a given column.

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Data Types A data type defines what kind of value a column can contain. Data type selection is usually dictated by nature of data and by intended use Pay close attention to expected use of attributes for sorting and data retrieval purposes




Data Types (Cont.)

Character CHAR(L)

VARCHAR(L) orVARCHAR2(L)

Fixed-length character data for up to 255 characters. If you store strings that are not as long as the CHAR parameter value, the remaining spaces are left unused. Therefore, if you specify CHAR(25), strings such as "Smith" and "Katzenjammer" are each stored as 25 characters. However, a U.S. area code is always three digits long, so CHAR(3) would be appropriate if you wanted to store such codes.

Variable-length character data. The designation VARCHAR2(25) will let you store characters up to 25 characters long. However, VARCHAR will not leave unused spaces. Oracle users may use VARCHAR2 as well as VARCHAR.

Date DATE Stores dates in the Julian date format.




SQL Constraints

SQL constraints are used to specify rules for the data in a table. NOT NULL - Indicates that a column cannot store NULL value UNIQUE - Ensures that each row for a column must have a unique value PRIMARY KEY - A combination of a NOT NULL and UNIQUE. Ensures that a column (or combination

of two or more columns) have an unique identity which helps to find a particular record in a table more easily and quickly. Most tables should have a primary key, and each table can have only ONE primary key.

FOREIGN KEY - Ensure the referential integrity of the data in one table to match values in another table. A FOREIGN KEY in one table points to a PRIMARY KEY in another table.

CHECK - Ensures that the value in a column meets a specific condition DEFAULT - Specifies a default value when specified none for this column




SQL Functions

SQL has many built-in functions for performing calculations on data. There are two types of SQL function:

SQL Aggregate functions SQL Scalar Functions




SQL Aggregate Functions

SQL aggregate functions return a single value, calculated from values in a column.

Useful aggregate functions:AVG() - Returns the average valueCOUNT() - Returns the number of rowsFIRST() - Returns the first valueLAST() - Returns the last valueMAX() - Returns the largest valueMIN() - Returns the smallest valueSUM() - Returns the sum




SQL Scalar functions

SQL scalar functions return a single value, based on the input value. Useful scalar functions:

UCASE() - Converts a field to upper caseLCASE() - Converts a field to lower caseMID() - Extract characters from a text fieldLEN() - Returns the length of a text fieldROUND() - Rounds a numeric field to the number of decimals specifiedNOW() - Returns the current system date and timeFORMAT() - Formats how a field is to be displayed




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References


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overview of sql basics part 1 - by software outsourcing company india

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