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SQL Constraints

SQL Constraints Overview

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.

SQL Contraints could be column level or table level. Column level constraints are applied only to one column where as 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 in SQL

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.

NOT NULL Constraint 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:

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)

);

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:

ALTER TABLE CUSTOMERS

MODIFY SALARY  DECIMAL (18, 2) NOT NULL;

DEFAULT Constraint in SQL

The SQL DEFAULT constraint provides a default value to a column when the INSERT INTO statement does not provide a specific value.

DEFAULT Constraint 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 INTPO statement does not provide a value for this column then by default this column would be set to 5000.00.

CREATE TABLE CUSTOMERS(

ID             INT                           NOT NULL,

NAME     VARCHAR (20)     NOT NULL,

AGE          INT                            NOT NULL,

ADDRESS  CHAR (25) ,

SALARY   DECIMAL (18, 2) DEFAULT 5000.00,

PRIMARY KEY (ID)

);

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:

ALTER TABLE CUSTOMERS

SALARY  DECIMAL (18, 2) DEFAULT 5000.00;

SQL Drop Default Constraint

To drop a DEFAULT constraint, use the following SQL:

ALTER TABLE CUSTOMERS

ALTER COLUMN SALARY DROP DEFAULT;

SQL UNIQUE Constraint

The  SQL 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.

SQL Unique Constraint 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:

CREATE TABLE CUSTOMERS(

ID            INT                           NOT NULL,

NAME    VARCHAR (20)     NOT NULL,

AGE        INT                            NOT NULL UNIQUE,

ADDRESS  CHAR (25) ,

SALARY   DECIMAL (18, 2),

PRIMARY KEY (ID)

);

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:

ALTER TABLE CUSTOMERS

MODIFY AGE INT NOT NULL UNIQUE;

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

ALTER TABLE CUSTOMERS

ADD CONSTRAINT myUniqueConstraint UNIQUE(AGE, SALARY);

DROP a UNIQUE Constraint

To drop a UNIQUE constraint, use the following SQL:

ALTER TABLE CUSTOMERS

DROP CONSTRAINT myUniqueConstraint;

If you are using MySQL then you can use following syntax:

ALTER TABLE CUSTOMERS

DROP INDEX myUniqueConstraint;

PRIMARY Key

A primary key is a field in a table which uniquely identifies the each rows/records 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.

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)

);

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.

Primary key field from one table and insert it into the other table where it becomes a foreign key ie. 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, 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 and 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 rows before it sort out 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 folloiwng 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 a INDEX Constraint

To drop a INDEX constraint, use the following SQL:

ALTER TABLE CUSTOMERS

DROP INDEX idx_age;

Constraints can be specified when a table is created with the CREATE TABLE statement or you can use ALTER TABLE statment to create constraints even after the table is created.

Dropping Constraints

Any constraint  that  you  have  defined  can  be  dropped  using  the  ALTER  TABLE  command  with  the  DROP CONSTRAINT option.

For example, to drop the primary key constraint in the EMPLOYEES table, you can use the following command:

ALTER TABLE EMPLOYEES DROP CONSTRAINT EMPLOYEES_PK;

Some implementations may provide shortcuts for dropping certain constraints. For example, to drop the primary key constraint for a table in Oracle, you can use the following command:

ALTER TABLE EMPLOYEES DROP PRIMARY KEY;

Some implementations allow you to disable constraints. Instead of permanently dropping a  constraint from the database, you may want to temporarily disable the constraint, and then enable it later.

Integrity Constraints

Integrity constraints are used to ensure accuracy and consistency of data in a relational database. Data integrity is handled in a relational database through the concept of referential integrity.

There are many types of integrity constraints that play a role in referential integrity (RI). These constraints include Primary Key, Foreign Key, Unique Constraints and other constraints mentioned above

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