oracle Update and delete data

except DBMS Common queries in （SELECT）, Insert （INSERT） Out of operation , The database also has some operations, such as ： to update , And delete data .

1. Update data

To update （ modify ） Table data , have access to UPDATE sentence . Use UPDATE There are two ways ：

• Update specific rows in the table
• Update all rows in the table

Warning ： Don't omit WHERE Clause

In the use of UPDATE Special care must be taken when , Because it is very easy to update every row in the table by mistake .

UPDATE Examples of use ：

UPDATE customers
SET cust_email = '[email protected]'
WHERE cust_id = 10005;

When updating multiple columns , Only a single SET command , And separate each with a comma column = value Yes （ There is no need to specify a comma after the last column ）.

Tips ： stay UPDATE The subquery is used in the statement

stay UPDATE You can use subqueries in statements , Allow to use through SELECT The data retrieved by the statement updates the column .

To delete the value of a column , You can set it to NULL.

UPDATE customers
SET cust_email = NULL
WHERE cust_id = 10005;

2. Delete data

To delete from the table （ remove ） data , have access to DELETE sentence , There are two ways ：

• Delete specific rows from the table
• Delete all rows from the table

DELETE Do not accept column names or wildcards , He will delete the whole line , Instead of deleting columns .

Be careful ： Is the table content , Not a watch

DELETE Statement will delete rows from the table , Even all rows in the table , however DELETE Never delete the table itself .

3. Guidelines for updating and deleting data

• Never perform without WHERE Clause UPDATE or DELETE, Unless you really intend to update and delete each row .
• Ensure that each table has a primary key （ If you forget what this is , See page 15 Chapter ）, And whenever possible , Just use it as WHERE Clause （ You may specify each primary key 、 Multiple values or value ranges ）.
• In combination WHERE Clause and UPDATE or DELETE Before , First of all, make use of SELECT Test it , To ensure that it filters out the correct records —— Write incorrect WHERE Clauses are much easier .
• Referential integrity implemented using the database （ See also page 15 Chapter learn about it ）, If the rows to be deleted have data associated with them in other tables , that Oracle Deletion will not be allowed .

Warning ： Careful use

The most important thing is ,Oracle No, Undo（ revoke ） Button . Use it very carefully UPDATE and DELETE, Otherwise, you will find yourself updating and deleting the wrong data .