DDL and DML Concepts

Data Definition Language(DDL)

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 most basic items of DDL are the CREATE, ALTER, RENAME, DROP and TRUNCATE statements:

               1.   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. Data type syntax is variable from system to system; the only way to be sure is to consult the documentation for your own software.

        2.  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.

       3.      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.

       4.       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>;

This is where consistent constraint naming comes in handy, so you can just remember the PK or FK name rather than remembering the syntax for looking up the names in another table. The DROP TABLE statement gets rid of its own PK constraint, but won’t work until you separately drop any FK constraints (or child tables) that refer to this one. It also gets rid of all data that was contained in the table—and it doesn't even ask you if you really want to do this!

       5.       All of the information about objects in your schema is contained, not surprisingly, in a set of tables that is called the data dictionary. There are hundreds of these tables most database systems, but all of them will allow you to see information about your own tables, in many cases with a graphical interface. How you do this is entirely system-dependent.

For Example:

CREATE TABLE My_table(

                                my_field1   INT,

my_field2   VARCHAR(50),

my_field3   DATE         NOT NULL,

PRIMARY KEY (my_field1, my_field2)

);

ALTER TABLE My_table ADD my_field4 NUMBER(3) NOT NULL;

TRUNCATE TABLE My_table;

DROP TABLE My_table;

Data Manipulation Language(DML)

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 retrieves data rather than modifying it.

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

                INSERT INTO <table name>

                VALUES (<value 1>, ... <value n>);

The comma-delimited list of values must match the table structure exactly in the number of attributes and the data type of each attribute. Character type values are always enclosed in single quotes; number values are never in quotes; date values are often (but not always) in the format 'yyyy-mm-dd' (for example, '2006-11-30').

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

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

UPDATE <table name>

                SET <attribute> = <expression>

                WHERE <condition>;

The update expression can be a constant, any computed value, or even the result of a SELECT statement that returns a single row and a single column. If the WHERE clause is omitted, then the specified attribute is set to the same value in every row of the table (which is usually not what you want to do). You can also set multiple attribute values at the same time with a comma-delimited list of attribute=expression pairs.

     3.       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.

     4.       If you are using a large multi-user system, you may need to make your DML changes visible to the rest of the users of the database. Although this might be done automatically when you log out, you could also just type:

                COMMIT;

     5.       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.

For Example:

INSERT INTO My_table

                                (field1, field2, field3)

                VALUES

                                ('test', 'N', NULL);

UPDATE My_table

SET field1 = 'updated value'

                                WHERE field2 = 'N';

DELETE FROM My_table

                                WHERE field2 = 'N';

CREATE TABLE tbl_1(id INT);

                                INSERT INTO tbl_1(id) VALUES(1);

                                INSERT INTO tbl_1(id) VALUES(2);

COMMIT;

                                UPDATE tbl_1 SET id=200 WHERE id=1;

SAVEPOINT id_1upd;

                                UPDATE tbl_1 SET id=1000 WHERE id=2;

ROLLBACK TO id_1upd;

                                SELECT id FROM tbl_1;

START TRANSACTION;

                                UPDATE Account SET amount=amount-200 WHERE account_number=1234;

                                UPDATE Account SET amount=amount+200 WHERE account_number=2345;

IF ERRORS=0 COMMIT;

IF ERRORS<>0 ROLLBACK;