SQL中ALTER和UPDATE命令之间的区别

1. ALTER命令：

ALTER SQL命令是DDL(数据定义语言)语句。 ALTER用于更新数据库中表的结构(如添加，删除，修改数据库中表的属性)。

句法 ：

// add a column to the existing table

ALTER TABLE tableName

ADD columnName columnDefinition;

 

// drop a column from the existing table

ALTER TABLE tableName

DROP COLUMN columnName;



// rename a column in the existing table

ALTER TABLE tableName

RENAME COLUMN olderName TO newName;



// modify the datatype of an already existing column in the table

ALTER TABLE table_name

ALTER COLUMN column_name column_type;

2. UPDATE命令：

UPDATE SQL命令是DML(数据操作语言)语句。它用于处理任何现有列的数据。但是不能更改表的定义。

句法 ：

// table name that has to update

UPDATE tableName

// which columns have to update

SET column1 = value1, column2 = value2, ...,columnN=valueN.

// which row you have to update

WHERE condition

注意：如果没有WHERE子句，表中的所有记录将被更新。

SQL中ALTER和UPDATE命令之间的区别：

SR.NO	ALTER Command	UPDATE Command
1	ALTER command is Data Definition Language (DDL).	UPDATE Command is a Data Manipulation Language (DML).
2	Alter command will perform the action on structure level and not on the data level.	Update command will perform on the data level.
3	ALTER Command is used to add, delete, modify the attributes of the relations (tables) in the database.	UPDATE Command is used to update existing records in a database.
4	ALTER Command by default initializes values of all the tuple as NULL.	UPDATE Command sets specified values in the command to the tuples.
5	This command make changes with table structure.	This command makes changes with data inside the table.
6	Example : Table structure, Table Name, SP, functions etc.	Example : Change data in the table in rows or in column etc.