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If you want more columns to be returned, you simply list them, separated by commas. Alternatively, you can use *, which means all columns. However, for performance reasons, if you need only certain columns, you should list them separately instead of asking for them all. Using * is not advisable even if at a particular moment you do want all the columns for a query because in future you may add even more columns to the table, and your query would end up asking for more data than is needed. Finally, using * doesn t guarantee the order in which the columns are returned, as one may change the order of the columns in a table (although this is not likely to happen). For these reasons, we don t use * in this book. With your current department table, the following two statements return the same results: SELECT department_id, name, description FROM department WHERE department_id = 1; SELECT * FROM department WHERE department_id = 1;
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You can split an SQL query on more lines, if you prefer PostgreSQL won t mind. Tip
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CHAPTER 3 CREATING THE PRODUCT CATALOG: PART I
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If you don t want to place any condition on the query, simply remove the WHERE clause, and you ll get all the rows. The following SELECT statement returns all rows and all columns from the product table: SELECT * FROM product;
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If you are impatient and can t wait until later in the chapter, you can test the SQL queries right now by Tip
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using the pgAdmin III tool! After connecting to the hatshop database, choose Tools Query tool. This will open a window where you can type and execute SQL queries on your database. Be careful, though, because in the rest of the book we ll assume the data in your department table is the same as shown previously in the chapter.
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Unless a sorting order is specified, the order in which the rows are returned by a SELECT clause can t be determined. Moreover, executing the same query twice could generate different results! To sort the results, you use ORDER BY. The following query will return the list of departments sorted alphabetically by the department name: SELECT department_id, name, description FROM department ORDER BY department_id;
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The INSERT statement is used to insert a row of data into the table. Its syntax is as follows: INSERT INTO <table name> [(column list)] VALUES (column values)
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Although the column list is optional (in case you don t include it, column values are assigned to Tip columns in the order in which they appear in the table s definition), you should always include it. This ensures that changing the table definition doesn t break the existing INSERT statements.
The following INSERT statement adds a department named Seasonal Hats Department to the department table: INSERT INTO department (name) VALUES ('Seasonal Hats Department'); No value was specified for the description field because it was marked to allow NULLs in the department table. This is why you can omit specifying a value, if you want to. Also, you re allowed to omit specifying a department ID because the department_id column was created with the serial option, which means the database takes care of automatically generating a value for it when adding new records. However, you re allowed to manually specify a value, if you prefer.
CHAPTER 3 CREATING THE PRODUCT CATALOG: PART I
Because department_id is the primary key column, trying to add more records with the same ID Tip
would cause the database to generate an error. The database doesn t permit having duplicate values in the primary key field.
When letting PostgreSQL generate values for serial columns, you can obtain the last generated value using the currval function. Here s an example of how this works: INSERT INTO department (name) VALUES ('Some New Department'); SELECT currval('department_department_id_seq');
In PostgreSQL, ; is the delimiter between SQL commands. Tip
UPDATE
The UPDATE statement is used to modify existing data and has the following syntax: UPDATE <table name> SET <column name> = <new value> [, <column name> = <new value> ... ] [WHERE <restrictive condition>] The following query changes the name of the department with the ID of 43 to Cool Department. If there were more departments with that ID, all of them would have been modified, but because department_id is the primary key, you can t have more departments with the same ID. UPDATE department SET name='Cool Department' WHERE department_id = 43; Be careful with the UPDATE statement because it makes it easy to mess up an entire table. If the WHERE clause is omitted, the change is applied to every record of the table, which you usually don t want to happen. PostgreSQL will be happy to change all of your records; even if all departments in the table would have the same name and description, they would still be perceived as different entities because they have different department_id values.
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