Basic SQL statements: DDL and DML
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.
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.
CREATE TABLE [
,
...
];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.
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
ADD CONSTRAINT PRIMARY KEY [];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
ADD CONSTRAINT FOREIGN KEY []
REFERENCES [];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 ;
ALTER TABLE
DROP CONSTRAINT ;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!
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. 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
VALUES [, ... ];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.
The update statement is used to change values that are already in a table. UPDATE
SET =
WHERE ;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.
The delete statement does just that, for rows in a table. DELETE FROM
WHERE ;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.
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;
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;
What is DDL in SQL with examples?
Data Definition Language[DDL] is a subset of SQL and a part of DBMS[Database Management System]. DDL consist of Commands to commands like CREATE, ALTER, TRUNCATE and DROP. These commands are used to create or modify the tables in SQL.
What is a DDL statement in SQL?
DDL or Data Definition Language actually consists of the SQL commands that can be used to define the database schema. It simply deals with descriptions of the database schema and is used to create and modify the structure of database objects in the database.
What is an example of a DDL statement?
Common examples of DDL statements include CREATE, ALTER, and DROP.
What are the 3 DDL commands?
Data Definition Language [DDL] commands:
ALTER for alteration. Truncate to delete data from the table. DROP to drop a table. RENAME to rename a table.
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