Chapter 3. SQL Basics

• SQL is based on the relational model but doesn't implement it faithfully
• SQL is a free form language whose statement can :
• be in uppercase or lowercase.
• start in any column.
• an intrenative sql statement ends with a semicolon.

• an identifier is a name that lets you refer to an object(schema,database ,column,key).
• database name must be unique.
• table and view names must be unique.
• column ,key,index and constraint names must be unique .

• you can give the same name of column in diferent tables.
• here's some more advice for choosing your identifier names :
• use a lowercase letters.
• names_with_underscore are easier to read than namewithunderscore.
• if you are worried that your identifier might be a reserved word in some other SQL dialect just add an underscore (for example element_).

• use the statement create table and alter table to defineor change a column data type.
• database designers choose data type carefully.
• use charactere string data types to represent text ,a character string or just string
  
• its lenght can be fixed or varying.
• it's a case sensitive('A' come before 'a').
• in sql statement ,a string is surrouded by a single quotes.

• charactere string types :
• CHARACTERE(lenght) represent a fixed numbers of characteres(CHARACTERE and CHAR are synonyms) .
• CHARACTERE VARYING (lenght) the DMBS store the string as is and doesn't pad itwith spaces.
• NATINAL CHARACTERE:this data type is the some as charactere except that is hold unicode charactere (NCHAR).
• CLOB:the caractere large object type is intended for use in library databases that holds vast among of text.
• NATIONAL CLOB:is like CLOB execpt that is hold unicode characteres.

• BINARY LARGE OBJECT TYPE: BLOB are used to store large amounts of multimedia data(graphic,audio,video,photo...),scientific data.
• BLOB can't be used as keys or indexes
• BLOB can be compared for only equality (=)or inequality(<>).
• you can't used BLOB with DISTINCT or GROUP BY or ORDER BY clauses.
• DMBS BLOB types :sql server binary,varbinary,image.

• EXACT NUMERIC TYPE :to represent exact numerical value
• it can be negative or positive number.
• it's an integer or a decimal number.
• it has a fixed precision and scale.
• exact numeric type:
• NUMERIC :represent a decimal number ,storedin a column defined as NUMERIC(precision,scale).
• DECIMAL is similaire to numeric ,the diference is that decimal has a precision greater that numeric.
• integer
  
• smallint: is similaire to integer but it might hold a smaller range of values.
• bigint
• sql server exact numeric type :bigint,iny ,smallint,tinyint,decimal,numeric.

• APPROXIMATE NUMERIC TYPES:an approximate numeric type has thesecharactéristics:
• it can a negative or positive number
• it typically is used to represent the very small or very large quantities common intechnical ,scientific calculations.
• approximate numeric type:
• FLOAT:represent a floating point number,stored in a column defined as FLOAT(precision).
• REAL:is similaire to float exceptthat the DMBS defines the precision(real takes no argument).
• DOUBLE PRECISION

• sql server :float,real.

• BOOLEAN TYPE:have three values :true ,false,unknown.
• sql server :bit,

• DATA TIME TYPE:use the data time type to represent data and time.
• you can compare two datatimevalues.
• sql server:datetime,smalldatetime,rowversion.

• OTHER DATA TYPE:the sql standard define other data type than the one covered in the preceding section.
• you can find data type for:network and internet @,links to files stored outside the databases.
• sql server:uniqueidentifier,identity.

• NULLS :when your data is imcomplete you can use a null to represent a missing or unknown values.
• NULLs are eliminated by splitting the original tableinto one-to-one relationship.