Difference Between Update and Alter in SQL

Update statement is used to update existing records in a database. Update is a Data Manipulation Language (DML) statement.

UPDATE        tableName
SET           column1Name=value1, column2Name=value2,
WHERE         columnXName=someValue

Alter SQL command is used to modify, delete or add a column to an existing table in a database. Alter is a Data Definition Language (DDL) statement.

ALTER TABLE   tableName
ADD           newColumnName dataTypeOfNewColumn
ALTER TABLE   tableName
DROP COLUMN   columnName
ALTER TABLE   tableName
ALTER COLUMN  columnName newDataType

Alter command modifies the database schema, while update statement only modifies records in a database without modifying its structure.

Read more: http://www.differencebetween.com/difference-between-update-and-vs-alter/#ixzz244ZO0N9T