SQL Constraints ? What is NULL value?

SQL Constraints :-

Constraints are the rules enforced on data columns on table. These are used to limit the type of data that can go into a table. This ensures the accuracy and reliability of the data in the database.

Constraints could be column level or table level. Column level constraints are applied only to one column, whereas table level constraints are applied to the whole table.

Following are commonly used constraints available in SQL:
 NOT NULL Constraint: Ensures that a column cannot have NULL value.
 DEFAULT Constraint: Provides a default value for a column when none is specified.
 UNIQUE Constraint: Ensures that all values in a column are different.
 PRIMARY Key: Uniquely identified each rows/records in a database table.
 FOREIGN Key: Uniquely identified a rows/records in any another database table.
 CHECK Constraint: The CHECK constraint ensures that all values in a column satisfy certain          conditions.

 INDEX: Use to create and retrieve data from the database very quickly


NOT NULL Constraint: 
By default, a column can hold NULL values. If you do not want a column to have a NULL value, then you need to
define such constraint on this column specifying that NULL is now not allowed for that column.
A NULL is not the same as no data, rather, it represents unknown data.
Example:
For example, the following SQL creates a new table called CUSTOMERS and adds five columns, three of which,
ID and NAME and AGE, specify not to accept NULLs:

If CUSTOMERS table has already been created, then to add a NOT NULL constraint to SALARY column in Oracle and MySQL, you would write a statement similar to the following:

DEFAULT Constraint: 
The DEFAULT constraint provides a default value to a column when the INSERT INTO statement does not provide a specific value.
Example:- For example, the following SQL creates a new table called CUSTOMERS and adds five columns . Here, SALARY column is set to 5000.00 by default, so in case INSERT INTO statement does not provide a value for this column.then by default this column would be set to 5000.00.

If CUSTOMERS table has already been created, then to add a DFAULT constraint to SALARY column, you would write a statement similar to the following:

Drop Default Constraint:- To drop a DEFAULT constraint, use the following SQL:-

UNIQUE Constraint: 

The UNIQUE Constraint prevents two records from having identical values in a particular column. In the 

CUSTOMERS table, for example, you might want to prevent two or more people from having identical age.

Example:

For example, the following SQL creates a new table called CUSTOMERS and adds five columns. Here, AGE column is set to UNIQUE, so that you can not have two records with same age:

If CUSTOMERS table has already been created, then to add a UNIQUE constraint to AGE column, you would write a statement similar to the following:-

You can also use following syntax, which supports naming the constraint in multiple columns as well:-

DROP a UNIQUE Constraint:

To drop a UNIQUE constraint, use the following SQL:-

If you are using MySQL, then you can use the following syntax:-

PRIMARY Key: 

A primary key is a field in a table which uniquely identifies each row/record in a database table. Primary keys must contain unique values. A primary key column cannot have NULL values.A table can have only one primary key, which may consist of single or multiple fields. When multiple fields are used as a primary key, they are called a composite key.If a table has a primary key defined on any field(s), then you can not have two records having the same value of that field(s).

Note:- You would use these concepts while creating database tables.

Create Primary Key :- 

Here is the syntax to define ID attribute as a primary key in a CUSTOMERS table.

To create a PRIMARY KEY constraint on the "ID" column when CUSTOMERS table already exists, use the following SQL syntax :-

ALTER TABLE CUSTOMER ADD PRIMARY KEY (ID);

NOTE :- If you use the ALTER TABLE statement to add a primary key, the primary key column(s) must already have 

been declared to not contain NULL values (when the table was first created).

For defining a PRIMARY KEY constraint on multiple columns, use the following SQL syntax :-

CREATE TABLE CUSTOMERS(

ID INT NOT NULL,

NAME VARCHAR (20) NOT NULL,

AGE INT NOT NULL,

ADDRESS CHAR (25) ,

SALARY DECIMAL (18, 2), 

PRIMARY KEY (ID, NAME)

);

To create a PRIMARY KEY constraint on the "ID" and "NAMES" columns when CUSTOMERS table already exists, 

use the following SQL syntax :-

ALTER TABLE CUSTOMERS 

ADD CONSTRAINT PK_CUSTID PRIMARY KEY (ID, NAME);

Delete Primary Key :-

You can clear the primary key constraints from the table, Use Syntax :-

ALTER TABLE CUSTOMERS DROP PRIMARY KEY ;

FOREIGN Key :-

A foreign key is a key used to link two tables together. This is sometimes called a referencing key.

Foreign Key is a column or a combination of columns whose values match a Primary Key in a different table.

The relationship between 2 tables matches the Primary Key in one of the tables with a Foreign Key in the 

second table.If a table has a primary key defined on any field(s), then you can not have two records having the same value of that field(s).

Example :- Consider the structure of the two tables as follows:

CUSTOMERS table :-

CREATE TABLE CUSTOMERS(

ID INT NOT NULL,

NAME VARCHAR (20) NOT NULL,

AGE INT NOT NULL,

ADDRESS CHAR (25) ,

SALARY DECIMAL (18, 2), 

PRIMARY KEY (ID)

);

ORDERS table :-

CREATE TABLE ORDERS (
ID INT NOT NULL,
DATE DATETIME, 
CUSTOMER_ID INT references CUSTOMERS(ID),
AMOUNT double,
PRIMARY KEY (ID)

);

If ORDERS table has already been created, and the foreign key has not yet been set, use the syntax for specifying a foreign key by altering a table.

ALTER TABLE ORDERS 
ADD FOREIGN KEY (Customer_ID) REFERENCES CUSTOMERS (ID);

DROP a FOREIGN KEY Constraint :-

To drop a FOREIGN KEY constraint, use the following SQL :-

ALTER TABLE ORDERS DROP FOREIGN KEY;

CHECK Constraint :- 
The CHECK Constraint enables a condition to check the value being entered into a record. If the condition
evaluates to false, the record violates the constraint and isn’t entered into the table.
Example :- For example, the following SQL creates a new table called CUSTOMERS and adds five columns. Here, we add a CHECK with AGE column, so that you can not have any CUSTOMER below 18 years :

CREATE TABLE CUSTOMERS(
ID INT NOT NULL,
NAME VARCHAR (20) NOT NULL,
AGE INT NOT NULL CHECK (AGE >= 18),
ADDRESS CHAR (25) ,
SALARY DECIMAL (18, 2), 
PRIMARY KEY (ID)
);

If CUSTOMERS table has already been created, then to add a CHECK constraint to AGE column, you would write a statement similar to the following :-

ALTER TABLE CUSTOMERS 
MODIFY AGE INT NOT NULL CHECK (AGE >= 18 );

You can also use following syntax, which supports naming the constraint in multiple columns as well :-

ALTER TABLE CUSTOMERS
ADD CONSTRAINT myCheckConstraint CHECK(AGE >= 18);

DROP a CHECK Constraint :-

To drop a CHECK constraint, use the following SQL. This syntax does not work with MySQL :-

ALTER TABLE CUSTOMERS
DROP CONSTRAINT myCheckConstraint;

INDEX :-

The INDEX is used to create and retrieve data from the database very quickly. Index can be created by using single or group of columns in a table. When index is created, it is assigned a ROWID for each row before it sortsout the data. Proper indexes are good for performance in large databases, but you need to be careful while creating index. Selection of fields depends on what you are using in your SQL queries.
Example :- For example, the following SQL creates a new table called CUSTOMERS and adds five columns :

CREATE TABLE CUSTOMERS(
ID INT NOT NULL,
NAME VARCHAR (20) NOT NULL,
AGE INT NOT NULL,
ADDRESS CHAR (25) ,
SALARY DECIMAL (18, 2), 
PRIMARY KEY (ID)
);

Now, you can create index on single or multiple columns using the following syntax :-

CREATE INDEX index_name

ON table_name ( column1, column2.....);

To create an INDEX on AGE column, to optimize the search on customers for a particular age, following is the SQL syntax :-

CREATE INDEX idx_age

ON CUSTOMERS ( AGE );

 DROP an INDEX Constraint :-

To drop an INDEX constraint, use the following SQL :-

ALTER TABLE CUSTOMERS
DROP INDEX idx_age;

What is NULL value?

A NULL value in a table is a value in a field that appears to be blank, which means a field with a NULL value is a  field with no value. It is very important to understand that a NULL value is different than a zero value or a field that contains spaces. A  field with a NULL value is one that has been left blank during record creation.