SQL-Alter Table, SELECT DISTINCT & WHERE
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The document explains SQL commands related to altering tables, selecting distinct values, and filtering records using the WHERE clause. It details how to add, drop, and modify columns in a table, as well as how to retrieve unique values and filter results based on conditions. Examples for each command are provided for clarity.
SQL-Alter Table, SELECT DISTINCT & WHERE
- 1. SQL ALTER TABLE, SELECT DISTINCT & WHERE VIDEO
- 2. ALTER TABLE • 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. • ADD COLUMN • DROP COLUMN • ALTER/MODIFY COLUMN
- 3. ADD COLUM N • ALTER TABLE table_name ADD column_name datatype; Syntax: • ALTER TABLE Customers ADD Email varchar(255); Example:
- 4. DROP COLUM N • ALTER TABLE table_name DROP COLUMN column_name; Syntax: • ALTER TABLE Customers DROP COLUMN Email; Example:
- 5. ALTER COLUM N • ALTER TABLE table_name ALTER COLUMN column_name datatype; Syntax: • ALTER TABLE Persons ALTER COLUMN DateOfBirth year; Example:
- 6. SELECT DISTINCT • The SELECT DISTINCT statement is used to return only distinct (different) values. • Inside a table, a column often contains many duplicate values; and sometimes you only want to list the different (distinct) values. Syntax: SELECT DISTINCT column1, column2, ... FROM table_name; Example:
- 7. WHERE • The WHERE clause is used to filter records. • It is used to extract only those records that fulfill a specified condition. Syntax: SELECT column1, column2, ... FROM table_name WHERE condition; Example: SELECT * FROM Customers WHERE Country='Mexico'; SELECT * FROM Customers WHERE CustomerID=1;