SQL Constraints - for database practices PDF

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SQL Constraints SQL constraints are used to specify rules for the data in a table. Constraints 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 table. If there is any violation between the constraint and the data action, the action is aborted. Constraints can be column level or table level. Column level constraints apply to a column, and table level constraints apply to the whole table. The following constraints are commonly used in SQL:       

NOT NULL - Ensures that a column cannot have a NULL value UNIQUE - Ensures that all values in a column are different PRIMARY KEY - A combination of a NOT NULL and UNIQUE. Uniquely identifies each row in a table FOREIGN KEY - Prevents actions that would destroy links between tables CHECK - Ensures that the values in a column satisfies a specific condition DEFAULT - Sets a default value for a column if no value is specified CREATE INDEX - Used to create and retrieve data from the database very quickly

SQL Constraints SQL constraints are used to specify rules for the data in a table. Constraints 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 table. If there is any violation between the constraint and the data action, the action is aborted. Constraints can be column level or table level. Column level constraints apply to a column, and table level constraints apply to the whole table. The following constraints are commonly used in SQL:     

NOT NULL - Ensures that a column cannot have a NULL value UNIQUE - Ensures that all values in a column are different PRIMARY KEY - A combination of a NOT NULL and UNIQUE. Uniquely identifies each row in a table FOREIGN KEY - Prevents actions that would destroy links between tables CHECK - Ensures that the values in a column satisfies a specific condition

 

DEFAULT - Sets a default value for a column if no value is specified CREATE INDEX - Used to create and retrieve data from the database very quickly

SQL UNIQUE Constraint The UNIQUE constraint ensures that all values in a column are different. Both the UNIQUE and PRIMARY KEY constraints provide a guarantee for uniqueness for a column or set of columns. A PRIMARY KEY constraint automatically has a UNIQUE constraint. However, you can have many UNIQUE constraints per table, but only one PRIMARY KEY constraint per table.

SQL UNIQUE Constraint on CREATE TABLE The following SQL creates a UNIQUE constraint on the "ID" column when the "Persons" table is created: SQL Server / Oracle / MS Access: CREATE TABLE Persons ( ID int NOT NULL UNIQUE, LastName varchar(255) NOT NULL, FirstName varchar(255), Age int ); MySQL: CREATE TABLE Persons ( ID int NOT NULL, LastName varchar(255) NOT NULL, FirstName varchar(255), Age int, UNIQUE (ID) );

To name a UNIQUE constraint, and to define a UNIQUE constraint on multiple columns, use the following SQL syntax: MySQL / SQL Server / Oracle / MS Access: CREATE TABLE Persons ( ID int NOT NULL, LastName varchar(255) NOT NULL, FirstName varchar(255), Age int, CONSTRAINT UC_Person UNIQUE (ID,LastName) );

SQL UNIQUE Constraint on ALTER TABLE To create a UNIQUE constraint on the "ID" column when the table is already created, use the following SQL: MySQL / SQL Server / Oracle / MS Access: ALTER TABLE Persons ADD UNIQUE (ID); To name a UNIQUE constraint, and to define a UNIQUE constraint on multiple columns, use the following SQL syntax: MySQL / SQL Server / Oracle / MS Access: ALTER TABLE Persons ADD CONSTRAINT UC_Person UNIQUE (ID,LastName);

DROP a UNIQUE Constraint To drop a UNIQUE constraint, use the following SQL:

MySQL: ALTER TABLE Persons DROP INDEX UC_Person; SQL Server / Oracle / MS Access: ALTER TABLE Persons DROP CONSTRAINT UC_Person;

SQL PRIMARY KEY Constraint The PRIMARY KEY constraint uniquely identifies each record in a table. Primary keys must contain UNIQUE values, and cannot contain NULL values. A table can have only ONE primary key; and in the table, this primary key can consist of single or multiple columns (fields).

SQL PRIMARY KEY on CREATE TABLE The following SQL creates a PRIMARY KEY on the "ID" column when the "Persons" table is created: MySQL: CREATE TABLE Persons ( ID int NOT NULL, LastName varchar(255) NOT NULL, FirstName varchar(255), Age int, PRIMARY KEY (ID) ); SQL Server / Oracle / MS Access: CREATE TABLE Persons ( ID int NOT NULL PRIMARY KEY, LastName varchar(255) NOT NULL, FirstName varchar(255),

Age int ); To allow naming of a PRIMARY KEY constraint, and for defining a PRIMARY KEY constraint on multiple columns, use the following SQL syntax: MySQL / SQL Server / Oracle / MS Access: CREATE TABLE Persons ( ID int NOT NULL, LastName varchar(255) NOT NULL, FirstName varchar(255), Age int, CONSTRAINT PK_Person PRIMARY KEY (ID,LastName) ); Note: In the example above there is only ONE PRIMARY KEY (PK_Person). However, the VALUE of the primary key is made up of TWO COLUMNS (ID + LastName).

SQL PRIMARY KEY on ALTER TABLE To create a PRIMARY KEY constraint on the "ID" column when the table is already created, use the following SQL: MySQL / SQL Server / Oracle / MS Access: ALTER TABLE Persons ADD PRIMARY KEY (ID); To allow naming of a PRIMARY KEY constraint, and for defining a PRIMARY KEY constraint on multiple columns, use the following SQL syntax: MySQL / SQL Server / Oracle / MS Access: ALTER TABLE Persons ADD CONSTRAINT PK_Person PRIMARY KEY (ID,LastName);

Note: If you use ALTER TABLE to add a primary key, the primary key column(s) must have been declared to not contain NULL values (when the table was first created).

DROP a PRIMARY KEY Constraint To drop a PRIMARY KEY constraint, use the following SQL: MySQL: ALTER TABLE Persons DROP PRIMARY KEY; SQL Server / Oracle / MS Access: ALTER TABLE Persons DROP CONSTRAINT PK_Person;

SQL FOREIGN KEY Constraint The FOREIGN KEY constraint is used to prevent actions that would destroy links between tables. A FOREIGN KEY is a field (or collection of fields) in one table, that refers to the PRIMARY KEY in another table. The table with the foreign key is called the child table, and the table with the primary key is called the referenced or parent table. Look at the following two tables:

Persons Table PersonID

LastName

FirstName

1

Hansen

Ola

2

Svendson

Tove

3

Pettersen

Kari

Orders Table OrderID

OrderNumber

PersonID

1

77895

3

2

44678

3

3

22456

2

4

24562

1

Notice that the "PersonID" column in the "Orders" table points to the "PersonID" column in the "Persons" table. The "PersonID" column in the "Persons" table is the PRIMARY KEY in the "Persons" table. The "PersonID" column in the "Orders" table is a FOREIGN KEY in the "Orders" table. The FOREIGN KEY constraint prevents invalid data from being inserted into the foreign key column, because it has to be one of the values contained in the parent table.

SQL FOREIGN KEY on CREATE TABLE The following SQL creates a FOREIGN KEY on the "PersonID" column when the "Orders" table is created: MySQL: CREATE TABLE Orders ( OrderID int NOT NULL, OrderNumber int NOT NULL, PersonID int, PRIMARY KEY (OrderID), FOREIGN KEY (PersonID) REFERENCES Persons(PersonID) ); SQL Server / Oracle / MS Access: CREATE TABLE Orders ( OrderID int NOT NULL PRIMARY KEY, OrderNumber int NOT NULL, PersonID int FOREIGN KEY REFERENCES Persons(PersonID) ); To allow naming of a FOREIGN KEY constraint, and for defining a FOREIGN KEY constraint on multiple columns, use the following SQL syntax: MySQL / SQL Server / Oracle / MS Access: CREATE TABLE Orders ( OrderID int NOT NULL, OrderNumber int NOT NULL, PersonID int, PRIMARY KEY (OrderID), CONSTRAINT FK_PersonOrder FOREIGN KEY (PersonID) REFERENCES Persons(PersonID) );

SQL FOREIGN KEY on ALTER TABLE

To create a FOREIGN KEY constraint on the "PersonID" column when the "Orders" table is already created, use the following SQL: MySQL / SQL Server / Oracle / MS Access: ALTER TABLE Orders ADD FOREIGN KEY (PersonID) REFERENCES Persons(PersonID); To allow naming of a FOREIGN KEY constraint, and for defining a FOREIGN KEY constraint on multiple columns, use the following SQL syntax: MySQL / SQL Server / Oracle / MS Access: ALTER TABLE Orders ADD CONSTRAINT FK_PersonOrder FOREIGN KEY (PersonID) REFERENCES Persons(PersonID);

DROP a FOREIGN KEY Constraint To drop a FOREIGN KEY constraint, use the following SQL: MySQL: ALTER TABLE Orders DROP FOREIGN KEY FK_PersonOrder; SQL Server / Oracle / MS Access: ALTER TABLE Orders DROP CONSTRAINT FK_PersonOrder;

SQL CHECK Constraint The CHECK constraint is used to limit the value range that can be placed in a column. If you define a CHECK constraint on a column it will allow only certain values for this column. If you define a CHECK constraint on a table it can limit the values in certain columns based on values in other columns in the row.

SQL CHECK on CREATE TABLE The following SQL creates a CHECK constraint on the "Age" column when the "Persons" table is created. The CHECK constraint ensures that the age of a person must be 18, or older: MySQL: CREATE TABLE Persons ( ID int NOT NULL, LastName varchar(255) NOT NULL, FirstName varchar(255), Age int, CHECK (Age>=18) ); SQL Server / Oracle / MS Access: CREATE TABLE Persons ( ID int NOT NULL, LastName varchar(255) NOT NULL, FirstName varchar(255), Age int CHECK (Age>=18) ); To allow naming of a CHECK constraint, and for defining a CHECK constraint on multiple columns, use the following SQL syntax: MySQL / SQL Server / Oracle / MS Access: CREATE TABLE Persons ( ID int NOT NULL, LastName varchar(255) NOT NULL, FirstName varchar(255), Age int, City varchar(255), CONSTRAINT CHK_Person CHECK (Age>=18 AND City='Sandnes') );

SQL CHECK on ALTER TABLE To create a CHECK constraint on the "Age" column when the table is already created, use the following SQL: MySQL / SQL Server / Oracle / MS Access: ALTER TABLE Persons ADD CHECK (Age>=18); To allow naming of a CHECK constraint, and for defining a CHECK constraint on multiple columns, use the following SQL syntax: MySQL / SQL Server / Oracle / MS Access: ALTER TABLE Persons ADD CONSTRAINT CHK_PersonAge CHECK (Age>=18 AND City='Sandnes');

DROP a CHECK Constraint To drop a CHECK constraint, use the following SQL: SQL Server / Oracle / MS Access: ALTER TABLE Persons DROP CONSTRAINT CHK_PersonAge; MySQL: ALTER TABLE Persons DROP CHECK CHK_PersonAge;

SQL Injection SQL injection is a code injection technique that might destroy your database. SQL injection is one of the most common web hacking techniques.

SQL injection is the placement of malicious code in SQL statements, via web page input.

SQL in Web Pages SQL injection usually occurs when you ask a user for input, like their username/userid, and instead of a name/id, the user gives you an SQL statement that you will unknowingly run on your database. Look at the following example which creates a SELECT statement by adding a variable (txtUserId) to a select string. The variable is fetched from user input (getRequestString):

Example txtUserId = getRequestString("UserId"); txtSQL = "SELECT * FROM Users WHERE UserId = " + txtUserId; The rest of this chapter describes the potential dangers of using user input in SQL statements.

SQL Injection Based on 1=1 is Always True Look at the example above again. The original purpose of the code was to create an SQL statement to select a user, with a given user id. If there is nothing to prevent a user from entering "wrong" input, the user can enter some "smart" input like this: 105 OR 1=1

UserId: Then, the SQL statement will look like this: SELECT * FROM Users WHERE UserId = 105 OR 1=1;

The SQL above is valid and will return ALL rows from the "Users" table, since OR 1=1 is always TRUE. Does the example above look dangerous? What if the "Users" table contains names and passwords? The SQL statement above is much the same as this: SELECT UserId, Name, Password FROM Users WHERE UserId = 105 or 1=1; A hacker might get access to all the user names and passwords in a database, by simply inserting 105 OR 1=1 into the input field.

SQL Injection Based on ""="" is Always True Here is an example of a user login on a web site: Username: John Doe

Password: myPass

Example uName = getRequestString("username"); uPass = getRequestString("userpassword"); sql = 'SELECT * FROM Users WHERE Name ="' + uName + '" AND Pass ="' + uPass + '"'

Result SELECT * FROM Users WHERE Name ="John Doe" AND Pass ="myPass"

A hacker might get access to user names and passwords in a database by simply inserting " OR ""=" into the user name or password text box: User Name:

Password:

The code at the server will create a valid SQL statement like this:

Result SELECT * FROM Users WHERE Name ="" or ""="" AND Pass ="" or ""="" The SQL above is valid and will return all rows from the "Users" table, since OR ""="" is always TRUE.

SQL Injection Based on Batched SQL Statements Most databases support batched SQL statement. A batch of SQL statements is a group of two or more SQL statements, separated by semicolons. The SQL statement below will return all rows from the "Users" table, then delete the "Suppliers" table.

Example SELECT * FROM Users; DROP TABLE Suppliers Look at the following example:

Example txtUserId = getRequestString("UserId"); txtSQL = "SELECT * FROM Users WHERE UserId = " + txtUserId; And the following input: 105; DROP

User id: The valid SQL statement would look like this:

Result SELECT * FROM Users WHERE UserId = 105; DROP TABLE Suppliers;

Use SQL Parameters for Protection To protect a web site from SQL injection, you can use SQL parameters. SQL parameters are values that are added to an SQL query at execution time, in a controlled manner.

ASP.NET Razor Example txtUserId = getRequestString("UserId"); txtSQL = "SELECT * FROM Users WHERE UserId = @0"; db.Execute(txtSQL,txtUserId); Note that parameters are represented in the SQL statement by a @ marker. The SQL engine checks each parameter to ensure that it is correct for its column and are treated literally, and not as part of the SQL to be executed.

Another Example txtNam txtAdd txtCit txtSQL

= = = =

getRequestString("CustomerName"); getRequestString("Address"); getRequestString("City"); "INSERT INTO Customers (CustomerName,Address,City)

Values(@0,@1,@2)"; db.Execute(txtSQL,txtNam,txtAdd,txtCit);

Examples The following examples shows how to build parameterized queries in some common web languages. SELECT STATEMENT IN ASP.NET: txtUserId = getRequestString("UserId"); sql = "SELECT * FROM Customers WHERE CustomerId = @0"; command = new SqlCommand(sql); command.Parameters.AddWithValue("@0",txtUserId); command.ExecuteReader(); INSERT INTO STATEMENT IN ASP.NET: txtNam = getRequestString("CustomerName"); txtAdd = getRequestString("Address"); txtCit = getRequestString("City"); txtSQL = "INSERT INTO Customers (CustomerName,Address,City) Values(@0,@1,@2)"; command = new SqlCommand(txtSQL); command.Parameters.AddWithValue("@0",txtNam); command.Parameters.AddWithValue("@1",txtAdd); command.Parameters.AddWithValue("@2",txtCit); command.ExecuteNonQuery(); INSERT INTO STATEMENT IN PHP: $stmt = $dbh->prepare("INSERT INTO Customers (CustomerName,Address,City) VALUES (:nam, :add, :cit)"); $stmt->bindParam(':nam', $txtNam); $stmt->bindParam(':add', $txtAdd); $stmt->bindParam(':cit', $txtCit); $stmt->execute();...