Understanding SQL Constraints and Joins

SQL Constraints

SQL constraints are rules enforced on data columns in a table. These are used to limit the type of data that can go into a table, ensuring the accuracy and reliability of the data.

Primary Key

A primary key is a field in a table which uniquely identifies each record in a database table. Primary keys must contain unique values, and cannot contain null values. A table can have only one primary key, which may consist of single or multiple fields.

Example of a primary key:

CREATE TABLE Customers (
    ID int NOT NULL,
    Name varchar(255) NOT NULL,
    Age int,
    PRIMARY KEY (ID)
);

Foreign Key

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.

Example of a foreign key:

CREATE TABLE Orders (
    OrderID int NOT NULL,
    OrderNumber int NOT NULL,
    CustomerID int,
    PRIMARY KEY (OrderID),
    FOREIGN KEY (CustomerID) REFERENCES Customers(ID)
);

SQL Joins

SQL Joins are used to combine rows from two or more tables, based on a related column between them.

Inner Join

The INNER JOIN keyword selects records that have matching values in both tables.

Example of an inner join:

SELECT Orders.OrderID, Customers.CustomerName
FROM Orders
INNER JOIN Customers
ON Orders.CustomerID = Customers.CustomerID;

Outer Join

The OUTER JOIN keyword returns records when there is a match in either the left (LEFT OUTER JOIN) or the right (RIGHT OUTER JOIN) table records. A FULL OUTER JOIN returns records when there is a match in either left (table1) or the right (table2) table records.

Example of a left outer join:

SELECT Customers.CustomerName, Orders.OrderID
FROM Customers
LEFT JOIN Orders
ON Customers.CustomerID = Orders.CustomerID;

By understanding and implementing SQL constraints and joins, you can ensure data integrity and effectively link data across multiple tables.

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