Maxxus PRO SPK-23 Especificaciones descargar pdf (Pagina 251)

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Let’s look at a few of the more common uses of ALTER TABLE.

One thing that comes up frequently is the realization that you haven’t made a particular col-

umn “big enough” for the data it has to hold. For example, in our Customers table, we have

allowed names to be 30 characters long. After we start getting some data, we might notice that

some of the names are too long and are being truncated. We can fix this by changing the data

type of the column so that it is 45 characters long instead:

alter table customers

modify name char(45) not null;

Another common occurrence is the need to add a column. Imagine that a sales tax on books is

introduced locally, and that Book-O-Rama needs to add the amount of tax to the total order,

but keep track of it separately. We can add a tax column to the Orders table as follows:

alter table orders

add tax float(6,2) after amount;

Getting rid of a column is another case that comes up frequently. We can delete the column we

just added as follows:

alter table orders

drop tax;

Deleting Records from the Database

Deleting rows from the database is very simple. You can do this using the DELETE statement,

which generally looks like this:

DELETE FROM table

[WHERE condition] [LIMIT number]

If you write

DELETE FROM table;

on its own, all the rows in a table will be deleted, so be careful! Usually, you want to delete

specific rows, and you can specify the ones you want to delete with a WHERE clause. You might

do this, if, for example, a particular book were no longer available, or if a particular customer

hadn’t placed any orders for a long time, and you wanted to do some housekeeping:

delete from customers

where customerid=5;

The LIMIT clause can be used to limit the maximum number of rows that are actually deleted.

Working with Your MySQL Database

HAPTER 9

ORKING WITH

YOUR MYSQL

DATABASE

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