SQL Data Definition Language: Create and Change the Database Ray Lockwood

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1 Introductory SQL SQL Data Definition Language: Create and Change the Database Pg 1 SQL Data Definition Language: Create and Change the Database Ray Lockwood Points: DDL statements create and alter the structure of a database. All interactions with the database (even when creating its structure) are done via SQL statements. Basic operations are SHOW, CREATE, ALTER, and DROP. DDL statements operate on Databases, Tables, and Columns. SQL is divided into two broad categories: Data Manipulation Language (DML) for reading and writing data in the tables, and Data Definition Language (DDL) for creating, altering, and deleting not just tables but the whole database schema (structure). Working with Databases To create a new database with tables in it, we follow these two steps: 1. Create the database 2. Create the tables A Database is Like a Folder When a Database Management System (DBMS) such as DB2, MySQL, or Oracle is installed, it may come with one or more databases already present. MySQL comes with three: two of them are for MySQL to keep up with its internal structures, and one is named test for the user to play with. DML works with the data. DDL works with the structure. The DBMS has the final say in how it lays things out. SDL merely makes hints which the DBMS might ignore. The system databases are critical to the DBMS. If they were deleted, the database would be destroyed. Each database is an isolated area, separate from the others, like a folder on a disk drive. The disk drive analogy is like this: Database DBMS Database Table File System File System Folder File Just like we create different folders in a filesystem for specific uses, we create different databases in the DBMS for different applications. List all databases Include the semicolon at the end or the query will continue to the next line: SHOW DATABASES; SHOW works the same for many objects.

2 Introductory SQL SQL Data Definition Language: Create and Change the Database Pg 2 Create a new, empty database CREATE DATABASE mydb; Delete a database Don t delete anything important; there's no undo! DROP DATABASE mydb; Change the current default database If you create a mydb database, you can make it the default database, just like going into a folder on your desktop makes it the default folder: USE mydb; Be real careful with this!! This will save you a lot of typing CREATE works the same for many objects. DROP destroys entire database objects. Don t confuse it with DELETE, which removes rows. USE sets the current database so you don t have to tell each command what database to use. Working with Tables Now that you ve created a database, and you re in it (it s the default database) you can work with your tables. See what tables are in a database SHOW TABLES; SHOW TABLES IN mydb; SHOW TABLES LIKE 'emp%'; SHOW TABLES IN mydb LIKE '%mp%'; All the tables in the current database All the tables in the named database Don t forget to use LIKE when you use wildcards Wildcards can appear anywhere in a string SHOW works the same for many objects. Wildcards SQL Windows % * _? The LIKE wildcard comparison operator works the same as in the SELECT statement.

3 Introductory SQL SQL Data Definition Language: Create and Change the Database Pg 3 Create a table CREATE TABLE Employee ( EmployeeNum INT, LastName CHAR(20) NOT NULL, FirstName CHAR(20) NOT NULL, DeptNum INT, Gender ENUM('m','f'), PRIMARY KEY (EmployeeNum) ); Constraints Enumerated data type Primary key constraint This statement creates the Employee table with five columns, each with its own data type. Then it designates a column as the primary key. The primary key constraint forbids the EmployeeNum column from accepting a NULL value, or allow a duplicate value in the column. (EmployeeNum is unique and non-null). Notice that the Primary Key Constraint is added separately from the creation of the attribute. Primary keys cannot contain NULL values. Create a table, multi-step using ALTER To help documentation, some shops create the columns in one statement, then set their constraints in separate ALTER statements: CREATE TABLE Employee ( EmployeeNum INT, LastName CHAR(20) NOT NULL, FirstName CHAR(20) NOT NULL, DeptNum INT, Gender ENUM('m','f') ); ADD PRIMARY KEY (EmployeeNum); See a CREATE statement for a table If you have an existing table and want to see a CREATE statement that will create such a table: SHOW CREATE TABLE Employee; Primary key declared separately to aid documentation. This is handy to either recreate the exact same empty table again, or just to jog your memory of how you created it. This is a matter of style You can ALTER a table even after you've started using it and it contains data. Even if you created your table in multiple steps, this shows all steps combined in a single CREATE statement. Rename a table RENAME TABLE Employee TO Emp; One of the few commands outside the basic four.

4 Introductory SQL SQL Data Definition Language: Create and Change the Database Pg 4 Delete a table This destroys data and there's no undo! DROP TABLE Employee; Copy a table to the same database First statement Second statement Handy when testing queries that change data: CREATE TABLE EmployeeBackup LIKE Employee; INSERT EmployeeBackup SELECT * FROM Employee; Copy a table to a different database Use the dot notation: Rows go from here... To here CREATE TABLE mydb.employeebackup LIKE HrDb.Employee; INSERT mydb.employeebackup SELECT * FROM HrDb.Employee; Create a new empty table Copy the rows into it This looses data! Copying a table is good for testing things on your database. It s a good idea to use a completely different server for testing. Database name Dot Table name Working with Columns List the columns List a table's columns, including their type and default value: SHOW COLUMNS IN Employee; Table name Can I change a column even when it contains data? You can change the columns in a table any time, even after you ve started using it and it contains data. Use SHOW COLUMNS to see how a table was built. Add a column ADD COLUMN MiddleInitial CHAR(1); Use ALTER TABLE to work with columns. Change a column's name Old name New name Existing Type (required) CHANGE COLUMN MiddleInitial MidInit CHAR(1); Even if you aren t changing the column s data type, you still have to include it.

5 Introductory SQL SQL Data Definition Language: Create and Change the Database Pg 5 Change a column's name and type Same syntax as above: CHANGE COLUMN MiddleInitial MiddleName VARCHAR(20); Same syntax as above Old name New name New type Change a column's type without renaming it You must include the column name twice in the first form: New type CHANGE COLUMN FirstName FirstName VARCHAR(20); Second form: MODIFY FirstName VARCHAR(20); Different syntax, same result Still the same syntax as above There're more than one way to do things. Delete a column This destroys any data already in the column, and there's no undo! DROP COLUMN MiddleInitial; This destroys data! Remove Data from a Table Remove a row from a table This is a regular DML statement: No WHERE clause DELETE FROM Employee WHERE EmployeeNum = 013 ; Remove all rows from a table Non-preferred Don t do this accidentally; there's no undo! DELETE From Employee; -- (non-preferred method) DELETE is a DML statement that deletes rows one by one, updating every index with each row a lengthy process! There s a better way the TRUNCATE statement which is DDL. Remove all rows from a table Preferred TRUNCATE eliminates all rows and renews the table structure by dropping and recreating it. It s much faster than DELETE, gives you fresh indexes, and is the preferred way to empty a table. But there's no undo! TRUNCATE TABLE Employee; -- (preferred method) WATCH OUT! If you forget the WHERE clause, you'll destroy ALL rows in the table! Removes the data and leaves the structure Better way to delete the data and preserve the structure. This is DML, not DDL. This incurs a lot of overhead and runs slowly. TRUNCATE doesn't record its actions in the logs, which we ll study later.

6 Introductory SQL SQL Data Definition Language: Create and Change the Database Pg 6 SQL Comments -- (double dash) is the SQL comment character. Everything from -- to the end of the line is ignored. /* */ defines a comment block, as it does in C and Java. MySQL Data Types Supported data types vary from one product to another, but they all provide a core set of types. MySQL supports 8 numeric types, 14 text types, and 5 date/time types. Important MySQL types are: Numeric Text Date/Time INT CHAR DATE BIGINT VARCHAR DATETIME FLOAT BINARY TIME DOUBLE BLOB YEAR DECIMAL LONGBLOB ENUM There are more Data Types than this. These are the common ones.

SQL: Concepts. Todd Bacastow IST 210: Organization of Data 2/17/ IST 210

SQL: Concepts. Todd Bacastow IST 210: Organization of Data 2/17/ IST 210 SQL: Concepts Todd Bacastow IST 210: Organization of Data 2/17/2004 1 Design questions How many entities are there? What are the major entities? What are the attributes of each entity? Is there a unique