KEYS

MySQL

Example

CREATE TABLE Orders (
    OrderID int,
    CustomerID int,
    OrderNumber int,
    PRIMARY KEY (OrderID)
);

Output

Query OK, 0 rows affected (0.20 sec)

Explanation

The CREATE TABLE statement creates a table named ‘Orders’ with three columns: OrderID, CustomerID, and OrderNumber. The PRIMARY KEY keyword denotes that the OrderID is the primary key of the Orders table. A primary key is a unique identifier for records in the table. It cannot contain NULL values and must contain unique values.

SQL Server

Example

CREATE TABLE Employees (
    ID int NOT NULL,
    Name varchar(255) NOT NULL,
    BirthDate date,
    Position varchar(255),
    Salary decimal(10,2),
    PRIMARY KEY (ID)
);

INSERT INTO Employees
VALUES (1, 'John Doe', '1980-10-01', 'Manager', 45000.00),
       (2, 'Jane Doe', '1982-05-09', 'Developer', 35000.00);

SELECT * FROM Employees;

Output

| ID |  Name   | BirthDate  | Position  | Salary  |
|----|---------|------------|-----------|---------|
| 1  |John Doe | 1980-10-01 | Manager   | 45000.00|
| 2  |Jane Doe | 1982-05-09 | Developer | 35000.00|

Explanation

In the SQL Server example given, an ‘Employees’ table is created with five columns: ‘ID’ (integer), ‘Name’ (string), ‘BirthDate’ (date), ‘Position’ (string), and ‘Salary’ (decimal). A primary key is set on the ‘ID’ column, which means that each entry in the ‘Employees’ table must have a unique ‘ID’. Two records are then inserted into the ‘Employees’ table, and finally, a query selects all records from the ‘Employees’ table and returns them as output.

Oracle

Example

CREATE TABLE Employees (
    ID INT PRIMARY KEY,
    NAME VARCHAR (20),
    AGE INT,
    SALARY DECIMAL (10, 2)
);

INSERT INTO Employees (ID, NAME, AGE, SALARY)
VALUES (1, "Mike", 45, 50000),
       (2, "Hannah", 35, 75000),
       (3, "Sam", 55, 97000);

Output

Query OK, 0 rows affected
Query OK, 3 rows affected

Explanation

The above SQL code is creating an Employees table with various columns: ID, NAME, AGE, and SALARY. The structure of the table includes a Primary Key ID to ensure there are no duplicate, null or conflicting entries. Then, three rows are inserted into this table. The Output demonstrates that the commands executed successfully without affecting any records (as this is a creation operation).