DROP DEFAULT
The DROP DEFAULT command is used to delete a DEFAULT constraint.
To drop a DEFAULT constraint, use the following SQL:
SQL Server / Oracle / MS Access:
ALTER TABLE Persons
ALTER COLUMN City DROP DEFAULT;
Category: SQL
SQL DROP DATABASE Keyword
DROP DATABASE
The DROP DATABASE command is used to delete an existing SQL database.
The following SQL drops a database named “testDB”:
Example
DROP DATABASE testDB;
Note: Be careful before dropping a database. Deleting a database will result in loss of complete information stored in the database!
SQL DROP CONSTRAINT Keyword
DROP CONSTRAINT
The DROP CONSTRAINT command is used to delete a UNIQUE, PRIMARY KEY, FOREIGN KEY, or CHECK constraint.
DROP a UNIQUE Constraint
To drop a UNIQUE constraint, use the following SQL :
SQL Server / Oracle / MS Access:
ALTER TABLE Persons
DROP CONSTRAINT UC_Person;
MySQL:
ALTER TABLE Persons
DROP INDEX UC_Person;
SQL DROP COLUMN Keyword
DROP COLUMN
The DROP COLUMN command is used to delete a column in an existing table.
The following SQL deletes the “ContactName” column from the “Customers” table:
Example
ALTER TABLE Customers
DROP COLUMN ContactName;
SQL DROP Keyword
DROP COLUMN
The DROP COLUMN command is used to delete a column in an existing table.
The following SQL deletes the “ContactName” column from the “Customers” table:
Example
ALTER TABLE Customers
DROP COLUMN ContactName;
SQL SELECT DISTINCT Keyword
SELECT DISTINCT
The SELECT DISTINCT command returns only distinct (different) values in the result set.
The following SQL statement selects only the DISTINCT values from the “Country” column in the “Customers” table : Continue reading SQL SELECT DISTINCT Keyword
SQL DESC Keyword
DESC
The DESC command is used to sort the data returned in descending order.
The following SQL statement selects all the columns from the “Customers” table, sorted descending by the “CustomerName” column : Continue reading SQL DESC Keyword
SQL DELETE Keyword
DELETE
The DELETE command is used to delete existing records in a table.
The following SQL statement deletes the customer “Alfreds Futterkiste” from the “Customers” table:
Example
DELETE FROM Customers WHERE CustomerName=‘Alfreds Futterkiste’;
Note: Be careful when deleting records in a table! Notice the WHERE clause in the DELETE statement. The WHERE clause specifies which record(s) should be deleted. If you omit the WHERE clause, all records in the table will be deleted!
It is possible to delete all rows in a table without deleting the table. This means that the table structure, attributes, and indexes will be intact:
The following SQL statement deletes all rows in the “Customers” table, without deleting the table. This means that the table structure, attributes, and indexes will be intact: Continue reading SQL DELETE Keyword
SQL DEFAULT Keyword
DEFAULT
The DEFAULT constraint provides a default value for a column.
The default value will be added to all new records if no other value is specified.
SQL DEFAULT on CREATE TABLE
The following SQL sets a DEFAULT value for the “City” column when the “Persons” table is created:
My SQL / SQL Server / Oracle / MS Access:
CREATE TABLE Persons (
City varchar(255) DEFAULT 'Sandnes'
);
The DEFAULT constraint can also be used to insert system values, by using functions like GETDATE(): Continue reading SQL DEFAULT Keyword
SQL DATABASE Keyword
CREATE DATABASE
The CREATE DATABASE command is used is to create a new SQL database.
The following SQL creates a database called “testDB”:
Example
CREATE DATABASE testDB;
Tip: Make sure you have admin privilege before creating any database. Once a database is created, you can check it in the list of databases with the following SQL command: SHOW DATABASES;