Studijní materiály

Zpět

Hromadně přidat materiály

Prezentace Structured Query Language_Nazarov Andrej

Y04A2L - Anglický jazyk 2-2

Hodnocení materiálu:

Zjednodušená ukázka:

Stáhnout celý tento materiál

y occur when the information stored in your database becomes out of date and you need to update it. UPDATE statement takes care of that (who could’ve thought (). Syntax looks like this:
UPDATE tablename SET columnname = newvalue WHERE condition.
The WHERE clause is again optional. So suppose you want to give your employees who worked more than 5 hours overtime a raise. This is how you’d do it using SQL:
UPDATE employees SET salary = salary * 1,25 WHERE overtime > 5
You can also modify the structure of an existing table – no need to copy and recreate another table. ALTER TABLE statement can do the work for you. With it you can add, delete (drop) and rename columns, set constraints, create primary keys and modify the data type of a column. Here is an almost self explanatory example:
ALTER TABLE sales
ADD seller_id integer
ADD contract_id NOT NULL PRIMARY KEY
DROP seller_name
MODIFY contact_phone varchar(13)
RENAME sum to total

Removing data
Time will come when you’ll need to get rid of certain entries when they’re not needed anymore. Two SQL statements are used for this occasion - DROP and DELETE. You can DROP a lot of things…from entire databases to users to tables. Syntax is easy to remember: DROP OBJECT_TYPE objectname. So for example to drop a table called meetings all you need to do is write DROP TABLE meetings. To remove a row (or several rows) matching a criteria you need to use the DELETE statement. Again it isn’t complicated. Looks like this basically: DELETE FROM tablename WHERE condition. So in order to remove all users who haven’t bought anything you can write:
DELETE FROM customers WHERE money_spent=0

As you can see the language is quite powerful and its data and database manipulation abilities are extensive. I haven’t even scratched the surface of this language and only mentioned the most important part – data manipulation. Most implementations of SQL are capable managing users on the server, creating complex constraints, logging all changes (to so-called transaction logs) and so on and so forth. And with this thought I’m going to end this presentation.

Stáhnout celý tento materiál

Předchozí

1 2 3

Vloženo: 28.04.2009

Velikost: 33,50 kB

Stáhnout celý tento materiál

Komentáře

Tento materiál neobsahuje žádné komentáře.

Mohlo by tě zajímat:

Skupina předmětu Y04A2L - Anglický jazyk 2-2
Reference vyučujících předmětu Y04A2L - Anglický jazyk 2-2

Podobné materiály