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Exercise: Creating PostgreSQL Types and Functions
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1. Start pgAdmin III, and connect to the hatshop database using the hatshopadmin username. 2. pgAdmin III has the interface elements that you can use to create types without writing code (you can check by right-clicking on Types, and selecting New Type). However, because the interface is not particularly friendly, we prefer to write and execute the code that performs the same actions. Choose Tools Query Tool, and write this code: CREATE TYPE department_list AS ( department_id INTEGER, name VARCHAR(50) ); 3. Press F5 to execute the command. The output should be similar to Query returned successfully with no result in 30 ms. 4. Choose Edit Clear Window to clear the current contents, and write the following code that creates the catalog_get_departments_list function: CREATE FUNCTION catalog_get_departments_list() RETURNS SETOF department_list LANGUAGE plpgsql AS $$ DECLARE outDepartmentListRow department_list; BEGIN FOR outDepartmentListRow IN SELECT department_id, name FROM department ORDER BY department_id LOOP RETURN NEXT outDepartmentListRow; END LOOP; END; $$;
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CHAPTER 3 CREATING THE PRODUCT CATALOG: PART I
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5. Press F5 to execute the command. The output should once again say that the command executed successfully with no result. 6. To test that the new function returns what it s supposed to, clear again the contents of the window, and type the following query. The list of departments should be retrieved (see Figure 3-13). SELECT * FROM catalog_get_departments_list();
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Figure 3-13. Executing a function using pgAdmin III 7. Close the Query tool window. If asked to save the changes, click No.
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How It Works: PostgreSQL Types and Functions
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Let s break down in parts the catalog_get_departments_list function. The first line is the one that defines the function name. Remember that you could use CREATE OR REPLACE FUNCTION if you have already created the function and want to change it. CREATE FUNCTION catalog_get_departments_list() The next line defines the return type and the language used in the function. The language we re using for this function, and for all the others in this book, is PL/pgSQL (LANGUAGE plpgsql). RETURNS SETOF department_list LANGUAGE plpgsql AS $$ The return type is SETOF department_list, which means the function is supposed to return one or more records that have the structure defined by the department_list type. The department_list type is a simple type composed of DepartmentID and Description, defined like this: CREATE TYPE department_list AS ( department_id INTEGER,
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CHAPTER 3 CREATING THE PRODUCT CATALOG: PART I
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VARCHAR(50)
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The body of the function is between the beginning and ending $$. The following code snippet represents the typical way we ll code our functions that return data. The bold line executes the query we re interested in, and the rest is auxiliary code required to return the results of that query. DECLARE outDepartmentListRow department_list; BEGIN FOR outDepartmentListRow IN SELECT department_id, name FROM department LOOP RETURN NEXT outDepartmentListRow; END LOOP; END; So what happens here The body of the function starts with the DECLARE section, which declares the variables that will be used by the function. Unlike with functions in other languages, a PL/pgSQL function has a special place where you can declare variables. In this case, the name of the variable is outDepartmentListRow, and its type is department_list. The code that performs the actual functionality is written between BEGIN and END. The syntax may look weird at first, but what it does is pretty straightforward. If you remember, the function is declared to return a set of values of the department_list type, and this is what it does. The function executes the SELECT statement, fetches each row of the results into the outDepartmentListRow variable (which is of the department_list type), and returns this variable. When this function finishes executing, it will have returned a set of department_list values.
Adding Logic to the Site
The business tier (or middle tier) is said to be the brains of the application because it manages the application s business logic. However, for simple tasks such as getting a list of departments from the data tier, the business tier doesn t have much logic to implement. It just requests the data from the database and passes it to the presentation tier. In this chapter, we re building the foundation of the business layer, which includes the functionality to open and close database connections, store SQL logic as PostgreSQL functions, and access these functions from PHP . For the business tier of the departments list, you ll implement two classes: DatabaseHandler will store the common functionality that you ll reuse whenever you need to access the database. Having this kind of generic functionality packed in a separate class saves keystrokes and avoids bugs in the long run. Catalog contains product catalog-specific functionality, such as the GetDepartments method that will retrieve the list of departments from the database.
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