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. Continue reading SQL FOREIGN 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).
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. Continue reading SQL UNIQUE Constraint
By default, a column can hold NULL values.
The NOT NULL constraint enforces a column to NOT accept NULL values.
This enforces a field to always contain a value, which means that you cannot insert a new record, or update a record without adding a value to this field.
SQL constraints are used to specify rules for data in a table.
Constraints can be specified when the table is created with the CREATE TABLE statement, or after the table is created with the ALTER TABLE statement. Continue reading SQL Constraints
The ALTER TABLE statement is used to add, delete, or modify columns in an existing table.
The ALTER TABLE statement is also used to add and drop various constraints on an existing table.
To add a column in a table, use the following syntax:
ALTER TABLE table_name
ADD column_name datatype;
The following SQL adds an “Email” column to the “Customers” table: Continue reading SQL ALTER TABLE Statement
The DROP TABLE statement is used to drop an existing table in a database.
DROP TABLE table_name;
Note: Be careful before dropping a table. Deleting a table will result in loss of complete information stored in the table!
The CREATE TABLE statement is used to create a new table in a database.
CREATE TABLE table_name (
column1 datatype,
column2 datatype,
column3 datatype,
....
);
The column parameters specify the names of the columns of the table.
The datatype parameter specifies the type of data the column can hold (e.g. varchar, integer, date, etc.).
Tip: For an overview of the available data types, go to our complete Data Types Reference. Continue reading SQL CREATE TABLE Statement
The BACKUP DATABASE statement is used in SQL Server to create a full back up of an existing SQL database.
BACKUP DATABASE databasename
TO DISK = 'filepath';
The DROP DATABASE statement is used to drop an existing SQL database.
DROP DATABASE databasename;
Note: Be careful before dropping a database. Deleting a database will result in loss of complete information stored in the database!