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CHAPTER 3 CREATING THE PRODUCT CATALOG: PART I
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Keep in mind that data type names are case insensitive, so you might see them capitalized differently depending on the database console program you re using. Now let s get back to the department table and determine which data types to use. Don t worry that you don t have the table yet in your database; you ll create it a bit later. Figure 3-9 shows the design of department in pgAdmin III. department_id is a serial data type, and name and description are varchar data types.
Figure 3-9. Designing the department table For character varying, the associated dimension such as in character varying(50) represents the maximum length of the stored strings. We ll choose to have 50 characters available for the department s name and 1,000 for the description. An integer record, as shown in the table, always occupies 4 bytes.
NOT NULL Columns and Default Values
For each column of the table, you can specify whether it is allowed to be NULL. The best and shortest definition for NULL is undefined. For example, in your department table, only department_id and name are really required, whereas description is optional meaning that you are allowed to add a new department without supplying a description for it. If you add a new row of data without supplying a value for columns that allow nulls, NULL is automatically supplied for them. Especially for character data, there is a subtle difference between the NULL value and an empty value. If you add a product with an empty string for its description, this means that you actually set a value for its description; it s an empty string, not an undefined (NULL) value. The primary key field never allows NULL values. For the other columns, it s up to you to decide which fields are required and which are not. In some cases, instead of allowing NULLs, you ll prefer to specify default values. This way, if the value is unspecified when creating a new row, it will be supplied with the default value. The default value can be a literal value (such as 0 for a salary column or "unknown" for a description column), a system value, or a function.
Serial Columns and Sequences
Serial columns are autonumbered columns. When a column is declared a serial column, PostgreSQL automatically provides values for it when inserting new records into the table. Usually if max is the largest value currently in the table for that column, then the next generated value will be max+1. This way, the generated values are always unique, which makes them especially useful when used in conjunction with the PRIMARY KEY constraint. You already know that primary keys
CHAPTER 3 CREATING THE PRODUCT CATALOG: PART I
are used on columns that uniquely identify each row of a table. If you set a primary key column to also be a serial column, PostgreSQL Server automatically fills that column with values when adding new rows (in other words, it generates new IDs),ensuring that the values are unique. Serial columns are defined using the serial data type. This data type is not a real data type but a notation that automatically defines a SEQUENCE structure over the integer data type. The following SQL code creates a table named department with a serial column that is also the primary key: CREATE TABLE department ( department_id SERIAL NOT NULL, name VARCHAR(50) NOT NULL, description VARCHAR(1000), CONSTRAINT pk_department_id PRIMARY KEY (department_id) ); This is in fact a shorter form of CREATE SEQUENCE department_department_id_seq; CREATE TABLE department ( department_id INTEGER NOT NULL DEFAULT nextval('department_department_id_seq'), name VARCHAR(50) NOT NULL, description VARCHAR(1000), CONSTRAINT pk_department PRIMARY KEY (department_id) ); When setting a serial column, the first value that PostgreSQL Server provides for that column is 1, but you can change this before adding data to your table with an SQL statement like the following: ALTER SEQUENCE department_department_id_seq RESTART WITH 123; This way, your PostgreSQL server will start generating values with 123. Now you understand that the default value shown in Figure 3-9 for department_id uses the sequence to generate new values for the column. For more details about the serial data type, see its official documentation at http://www.postgresql.org/docs/current/interactive/datatype.html#DATATYPE-SERIAL. The documentation for updating the sequence can be found at http://www.postgresql.org/ docs/current/interactive/sql-altersequence.html.
Note Unlike other database servers, PostgreSQL still allows you to manually specify for an autonumbered
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