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Basic SQL statements: DDL and DML
http://www.tomjewett.com/dbdesign/dbdesign.php?page=ddldml.php


In the first part of this tutorial, you’ve seen some of the SQL statements that you need to start building a database.
This page gives you a review of those and adds several more that you haven’t seen yet.

• SQL statements are divided into two major categories: data definition language (DDL)
and data manipulation language (DML).


Data definition language

DDL statements are used to build and modify the structure of your tables and other objects in the database.
When you execute a DDL statement, it takes effect immediately.

• The create table statement does exactly that:

        CREATE TABLE <table name> (
        <attribute name 1> <data type 1>,
        ...
        <attribute name n> <data type n>);

The data types that you will use most frequently are character strings,
which might be called VARCHAR or CHAR for variable or fixed length strings;
numeric types such as NUMBER or INTEGER, which will usually specify a precision;
and DATE or related types.

• The alter table statement may be used as you have seen to specify primary and foreign key constraints,
as well as to make other modifications to the table structure.
Key constraints may also be specified in the CREATE TABLE statement.

        ALTER TABLE <table name>
        ADD CONSTRAINT <constraint name> PRIMARY KEY (<attribute list>);

You get to specify the constraint name.
Get used to following a convention of tablename_pk
(for example, Customers_pk), so you can remember what you did later.

The attribute list contains the one or more attributes that form this PK;
if more than one, the names are separated by commas.

• The foreign key constraint is a bit more complicated,
since we have to specify both the FK attributes in this (child) table,
and the PK attributes that they link to in the parent table.

        ALTER TABLE <table name>
        ADD CONSTRAINT <constraint name> FOREIGN KEY (<attribute list>)
        REFERENCES <parent table name> (<attribute list>);

Name the constraint in the form childtable_parenttable_fk (for example, Orders_Customers_fk).
If there is more than one attribute in the FK, all of them must be included (with commas between)
in both the FK attribute list and the REFERENCES (parent table) attribute list.

You need a separate foreign key definition for each relationship in which this table is the child.

• If you totally mess things up and want to start over, you can always get rid of any
object you’ve created with a drop statement. The syntax is different for tables and constraints.

        DROP TABLE <table name>;

        ALTER TABLE <table name>
        DROP CONSTRAINT <constraint name>;



Data manipulation language

DML statements are used to work with the data in tables. 
When you are connected to most multi-user databases
(whether in a client program or by a connection from a Web page script),
you are in effect working with a private copy of your tables that can’t be seen by anyone else
until you are finished (or tell the system that you are finished).
You have already seen the SELECT statement;
it is considered to be part of DML even though it just retreives data rather than modifying it.

• The insert statement is used, obviously, to add new rows to a table.

        INSERT INTO <table name>
        VALUES (<value 1>, ... <value n>);


Yes, you will need a separate INSERT statement for every row.

• The update statement is used to change values that are already in a table.

        UPDATE <table name>
        SET <attribute> = <expression>
        WHERE <condition>;

You can also set multiple attribute values at the same time with a comma-delimited list of attribute=expression pairs.

• The delete statement does just that, for rows in a table.

        DELETE FROM <table name>
        WHERE <condition>;
If the WHERE clause is omitted, then every row of the table is deleted (which again is usually not what you want to do)—and again, you will not get a “do you really want to do this?” message.


        COMMIT;
• If you’ve messed up your changes in this type of system,
and want to restore your private copy of the database to the way it was
before you started (this only works if you haven’t already typed COMMIT), just type:

        ROLLBACK;
Although single-user systems don’t support commit and rollback statements,
they are used in large systems to control transactions, which are sequences of changes to the database.
Transactions are frequently covered in more advanced courses.

Privileges

If you want anyone else to be able to view or manipulate the data in your tables,
and if your system permits this, you will have to explicitly grant the appropriate privilege or privileges
(select, insert, update, or delete) to them.
This has to be done for each table.
The most common case where you would use grants is for tables
that you want to make available to scripts running on a Web server, for example:

        GRANT select, insert ON customers TO webuser;

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※ 作者: rikaka 時間: 2012-03-30 18:18:34