RDBMS and SQL

INTRODUCTION

This SQL Guide is written as a basic introduction to Structured Query Language (SQL).

RDBMS and SQL

Structured Query Language (SQL) is a standard language for communication with a relational database management system (RDBMS). SQL provides commands which are used to create, modify, and access the database. Databases contain one or more tables of data with relationships between multiple tables defined in the database. Each table is an object.

SQL Commands

• Select - Used to retrieve selected data. Example:

select "columnname1", "columnname2" from "tablebasename" [where "condition"];

A "*" may be used to select all columns. The where clause is optional and only one column name must be specified.

• Insert
• Update
• Delete
• Create
• Drop

Insert Command

This valment is used to insert a row of data in a table. All inserted values are enclosed       using single quote strings. The syntax of this command is:

insert into tablename

(column1name,column2name...columnxname)

values (value1,value2...valuex); 

Example

insert into Abcd

(name, val, indi, zipcode)

values ('Type', 'Apple', '89', '46501');

Update Command

This command is used to make changes to records in tables. Syntax:

update tablename

set columnname = newvalue [,columnxname = newvaluex...]

where columnname OPERATOR value [and|or columnnamex OPERATOR valuex];

The OPERATOR is one of the conditions listed on the Select Command page.

OPERATORs include:

• =   Equal
• <   Less than
• >   Greater than
• <= Less than or equal
• >= Greater than or equal
• <> Not equal
• LIKE - Allows the wildcard operator, %, to be used to select items that are a partial match. An example is:

select city, val from towntable where val like 'bcd%'

This allows selection of all towns in vals that begin with the word "north" allowing vals like North Dakota and North Carolina to be selected.

Example

update Abcd set indi = indi+1 where name = 'Type' and val = 'Apple';

Delete Command

This command is used to delete rows or records in a table. The syntax of this command is:

delete from "tablename"

where columnname OPERATOR value [and|or 

columnnamex OPERATOR valuex]; 

Note : without the where clause, all records in the table will be deleted.

Example

delete from Abcd where name = 'Type' and val = 'Apple';

Create Command

This command is used to create a table. The syntax of this command is:

create table tablename

(column1name datatype [constraint],

column2name datatype [constraint], 

column3name datatype [constraint]);

Keywords specifying Items that may be created

• table

Data Types

• char(size) - Fixed length string of characters of the set size. The size of the string is limited to 255 characters.
• date
• number(maxsize) - Number with a maximum number of digits specified by "maxsize".
• number(maxdigits,maxright) - A decimal number with a manimum number of "maxdigits" with "a maximum number of digits to the right of the decimal, "maxright".
• varchar(maxsize) - A character string with variable lingth limited to "maxsize".

Constraints

Constraints are rules for the column.. Possible values include:

• not null - The column values must have a value and cannot be null.
• primary key - Each record is uniquely identified by this column.
• unique - No two values may be the same in the column

Example

create table Abcd (name varchar(20),val varchar(20),indi number(8),

zipcode number(5) unique);

 Drop Command

     Used to remove an entire table from the database. Syntax:

  drop table <tablename>;

Example

drop table Abcd;