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Databases: A Beginner s Guide
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Figure 4-27 UPDATE statement for the EMPLOYEES table
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name, or any other expression that SQL can resolve to a column value If the SET clause references multiple columns, the column names and values must be in a comma-separated list The UPDATE statement may include a WHERE clause to limit the rows affected by the statement If the WHERE clause is omitted, the UPDATE statement will attempt to update every row in the table (or view) If you forget this key point, remember our friend the ROLLBACK statement, which can back out the results of the update (unless you are in autocommit mode, of course)
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The DELETE Statement
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The DELETE statement removes one or more rows from a table The statement can also reference a view, but only if the view is based on a single table (in other words, views that join multiple tables cannot be referenced) A DELETE statement does not reference columns because the statement automatically clears all column data for any rows deleted A WHERE clause can be included to limit the rows affected by the DELETE statement; if the WHERE clause is omitted, the statement attempts to delete all the rows in
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4: Introduction to SQL
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Figure 4-28 DELETE statement for the EMPLOYEES table
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the referenced table Figure 4-28 shows an example of a DELETE statement that deletes employee 921 from the EMPLOYEES table Here is the SQL statement:
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DELETE FROM EMPLOYEES WHERE EMPLOYEE_ID = 921;
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Data Definition Language (DDL) Statements
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Data Definition Language (DDL) statements define the database objects but do not insert or update any data stored within those objects (DML statements serve that purpose) In SQL, three basic commands are used within DDL:
CREATE
Creates a new database object of the type named in the statement
DROP Drops (destroys) an existing database object of the type named in the statement ALTER Changes the definition of an existing database object of the type named in the statement
Databases: A Beginner s Guide
The sections that follow examine the most commonly used DDL statement types You ll find a lot of variety in DDL statements available across RDBMS vendors, so consult the vendor s documentation for more details
The CREATE TABLE Statement
The CREATE TABLE statement adds a new table to the database Here is an example that creates the EMPLOYEE_INPUT table with the same column definitions as the EMPLOYEES table:
CREATE TABLE EMPLOYEE_INPUT ( EMPLOYEE_ID NUMBER(6) FIRST_NAME VARCHAR2(20) LAST_NAME VARCHAR2(25) EMAIL VARCHAR2(25) PHONE_NUMBER VARCHAR2(20) HIRE_DATE DATE JOB_ID VARCHAR2(10) SALARY NUMBER(8,2) COMMISSION_PCT NUMBER(2,2) MANAGER_ID NUMBER(6) DEPARTMENT_ID NUMBER(4) ; NOT NULL, NULL, NOT NULL, NOT NULL, NULL, NOT NULL, NOT NULL, NULL, NULL, NULL, NULL)
Note that a comma-separated list of columns is provided, along with the data type and NULL or NOT NULL specification for each You may recall from 2 that a wide variety of data types is supported across RDBMS vendors The data types shown here apply to Oracle Be careful with NULL and NOT NULL specifications In most RDBMSs, including Oracle, NULL is the default However, in others, the default might be NOT NULL It is therefore safer, but of course more work, always to specify either NULL or NOT NULL Incidentally, most RDBMSs require that primary key columns be specified as NOT NULL You ll see how to create a primary key constraint on the EMPLOYEE_ID column of this table in the Primary Key Constraints section a little further along in this chapter Many vendor extensions to the CREATE TABLE statement exist beyond the basic column list used in our example For example, in Oracle, the STORAGE clause can be included to specify the amount of physical space that is to be allocated to the table, and a TABLESPACE clause can be included to specify the tablespace that will hold the table s data
4: Introduction to SQL
The ALTER TABLE Statement
The ALTER TABLE statement can be used to change many aspects of the definition of a database table Again, a wide variation in implementation exists across RDBMS vendors, but generally speaking, the following types of changes can be made using the ALTER TABLE statement:
Adding columns to the table Removing columns from the table Altering the data type for existing table columns Changing physical storage attributes of the table Adding, removing, or altering constraints
Because the implementation of constraints is the way we enforce business rules in the database, we will take a closer look at them here It is important that you name the constraints, because in most SQL implementations the names appear in the error messages generated when constraint violations take place
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