1. CS4222 Principles of Database System
11/30/2018
CS4222 Principles of Database System
13. PL/PgSQL
Huiping Guo
Department of Computer Science
California State University, Los Angeles

2. Introduction PL/pgSQL is a loadable, procedural language.
11/30/2018
Introduction
PL/pgSQL is a loadable, procedural language.
A procedural language is a programming language used to specify a sequence of steps that are followed to produce an intended programmatic result
What PL/PgSQL is used for?
extending the server functionality via user-defined functions, triggers, etc
Advantages
group sequences of SQL and programmatic statements together within a database server
reduces network and communications overhead
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3. Adding PL/pgSQL to Database
To add PL/pgSQL you can either use the createlang application from command prompt, or the CREATE LANGUAGE SQL command from within a database client such as psql
C:\>createlang -U hpguo  -d hpguo plpgsql
CREATE LANGUAGE command first requires the creation of the PL/pgSQL call handler, which is the function that actually processes and interprets the PL/pgSQL code.
You don’t need to do this unless you installed your own server on your home machine!!
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4. Language Structure
PL/pgSQL's structure is similar to other programming languages such as C
each portion of code acts as a function
all variables must be declared before being used
code segments accept arguments when called and return arguments at their end
PL/pgSQL functions are case insensitive
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5. Code Blocks
PL/pgSQL code is organized in blocks of code (block structured code )
Code blocks are entered within a CREATE FUNCTION
CREATE FUNCTION command
names the new function
states its argument types
states the return type
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6. Structure of a PL/pgSQL code block
CREATE FUNCTION funName (arguments) RETURNS type AS ’
DECLARE
declaration;
BEGIN
statement;
END;
’ LANGUAGE 'plpgsql';
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7. Code Blocks (cont.)
function's main code block starts with a declaration section
variable declaration specifies the variable's name and type
the main body of the code block is started with the BEGIN keyword
END keyword designates the end of the code block
PL/pgSQL function should return a value of its specified return type before its END keyword is reached
All strings in the code blocks must be enclosed within ‘‘ ’’ (two single quotes)
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8. Comments C style SQL style /* This is a multiple-line comment. */
-- This is a single line comment.
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9. A simple example Define a function Use the function
CREATE FUNCTION a_function () RETURNS int4 AS '
DECLARE an_integer int4 :=1;
BEGIN
an_integer := 10 * 10;
return an_integer;
END;
' LANGUAGE 'plpgsql';
Use the function
Select a_function() as output;
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10. Declaration and statement
Declare and/or initialize the variables that will be referenced within the code block.
Each declaration is terminated by a semicolon
Statement
A statement performs an action within PL/pgSQL code
such as assignment of a value to a variable
the execution of a query, ….
Each statement within a block is terminated by a semicolon
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11. Declare a variable myrow sailors%ROWTYPE;
variable_name [ CONSTANT ] data_type [ NOT NULL ]
[ { DEFAULT | := } value ];
Ex: subject_id INTEGER;
five CONSTANT INTEGER := 5;
ten INTEGER NOT NULL := 10;
letter CHAR DEFAULT ‘’a’’;
myrow sailors%ROWTYPE;
myfield tablename.columnname %TYPE;
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12. Declare a variable (cont.)
five CONSTANT INTEGER := 5;
The variable five cannot be changed.
If you change five in the function, you’ll get run time error
ten INTEGER NOT NULL := 10;
The variable ten cannot be set to NULL
letter CHAR DEFAULT “a”;
equivalent to
letter CHAR :=’’a’’
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13. Example CREATE FUNCTION example_function () RETURNS text AS ' DECLARE
five CONSTANT INTEGER := 5;
ten INTEGER NOT NULL := 10;
letter CHAR DEFAULT ''a'';
BEGIN return letter;
END;
' LANGUAGE 'plpgsql';
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14. Data type SQL type Composite variable type
Aliases for Function Parameters
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15. Commonly used SQL data type
Boolean, bool
text
char
Integer, int, int4
Real, float8
date
time
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16. Composite variable type
Row type:
used to declare a PL/pgSQL record variable with the same structure as the rows in a table you specify.
myrow tablename%ROWTYPE;
Attribute type
used to declare a variable with the data type of a table column.
myfield tablename.columnname%TYPE;
Record type
Similar to row type, without predefined structure
Myrecord RECORD
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17. Aliases for Function Parameters
Parameters passed to functions are named with the identifiers $1, $2, etc.
Either the alias or the numeric identifier can then be used to refer to the parameter value.
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18. Aliases for Function Parameters
CREATE FUNCTION sales_tax(REAL) RETURNS REAL AS '
DECLARE
subtotal ALIAS FOR $1;
BEGIN
return subtotal * 0.06;
END;
' LANGUAGE 'plpgsql';
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19. Aliases for Function Parameters
CREATE FUNCTION instr(VARCHAR,INTEGER) RETURNS INTEGER AS ‘
DECLARE
v_string ALIAS FOR $1;
index ALIAS FOR $2;
BEGIN
-- Some computations here
END;
' LANGUAGE 'plpgsql';
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20. Statement Assignment Controlling Program Flow Basic assignment
SELECT INTO
PEROFRM
EXCUTE
Controlling Program Flow
Conditionals
Loops
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21. Basic assignment identifier := expression; Examples: user_id := 20;
tax := subtotal * 0.06;
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22. SELECT INTO Syntax target SELECT target_column [, ...]
INTO target_variable [, ...]
FROM table_name
WHERE conditions
target
record variable, a row variable, or a comma-separated list of simple variables and record/row fields.
takes the value of the first row returned
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23. SELECT INTO statement
Create table Customers (id int primary key, name text);
Insert into Customers Values(1, 'Tom');
Insert into Customers Values(2, 'Annie');
CREATE FUNCTION get_customer_id (text) RETURNS integer AS '
DECLARE
customer_name ALIAS FOR $1;
customer_id INTEGER;
BEGIN
SELECT id INTO customer_id FROM customers
WHERE name = customer_name;
RETURN customer_id;
END;
' LANGUAGE 'plpgsql';
Use the function: SELECT get_customer_id('Annie');
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24. SELECT INTO statement
CREATE FUNCTION get_customer_name (integer) RETURNS text AS '
DECLARE
customer_id ALIAS FOR $1;
customer_name TEXT;
BEGIN
SELECT name INTO customer_name
FROM customers
WHERE id = customer_id;
RETURN customer_name;
END;
' LANGUAGE 'plpgsql';
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25. Using the FOUND boolean
[...]
SELECT id INTO customer_id FROM customers
WHERE name = customer_name;
IF NOT FOUND THEN return -1;
END IF;
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26. Using %ROWTYPE
CREATE FUNCTION get_customer (integer) RETURNS text AS '
DECLARE
customer_id ALIAS FOR $1;
found_customer customers%ROWTYPE;
BEGIN
SELECT * INTO found_customer
FROM customers
WHERE id = customer_id;
RETURN found_customer.id || '' '' || found_customer.name;
END;
' LANGUAGE 'plpgsql';
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27. Using %TYPE CREATE FUNCTION get_customer2 (integer) RETURNS text AS '
DECLARE
customer_id ALIAS FOR $1;
customer_name customers.name%TYPE;
BEGIN
SELECT name INTO customer_name
FROM customers
WHERE id = customer_id;
RETURN customer_id || '' '' || customer_name;
END;
' LANGUAGE 'plpgsql';
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28. Contionals IF ... THEN ... ELSIF ... ELSE IF ... THEN
IF ... THEN ... ELSE
IF ... THEN ... ELSE IF
IF ... THEN ... ELSIF ... ELSE
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29. IF-THEN IF v_user_id <> 0 THEN
UPDATE users SET = v_ WHERE user_id = v_user_id;
END IF;
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30. IF-THEN-ELSE IF v_count > 0 THEN INSERT INTO users_count (count)
VALUES (v_count);
RETURN ''t'';
ELSE
RETURN ''f'';
END IF;
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31. IF-THEN-ELSE IF IF demo_row.sex = ''m'' THEN pretty_sex := ''man'';
ELSE IF demo_row.sex = ''f''
THEN pretty_sex := ''woman'';
END IF;
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32. IF-THEN-ELSIF-ELSE IF number = 0 THEN result := ''zero'';
ELSIF number > 0
THEN result := ''positive'';
ELSIF number < 0
THEN result := ''negative'';
ELSE result := ''NULL'';
END IF;
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33. LOOPS Simple loops While For Looping Through Query Results Exit
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34. Simple loops LOOP statement; [...] EXIT [ WHEN condition ]; END LOOP;
LOOP defines an unconditional loop that is repeated indefinitely until terminated by an EXIT or RETURN statement.
The innermost loop is terminated and the statement following END LOOP is executed next.
If WHEN is specified, the loop exit occurs only if expression is true. Otherwise, control passes to the statement after EXIT.
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35. Simple loops CREATE FUNCTION square_integer_loop(integer)
RETURNS integer AS '
DECLARE
num1 ALIAS FOR $1;
result integer;
BEGIN
result := num1;
LOOP
result := result * result;
EXIT WHEN result >= 10000
END LOOP;
RETURN result;
END;
' LANGUAGE 'plpgsql';
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36. Exit Ex.1 LOOP IF count > 0 THEN EXIT; -- exit loop END IF;
END LOOP;
Ex.2
EXIT WHEN count > 0; -- same result as previous example
IF stocks >
THEN EXIT; -- invalid; cannot use EXIT outside of LOOP
END;
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37. While WHILE condition LOOP statement; [...] END LOOP;
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38. While CREATE FUNCTION while_loop (integer, integer)
RETURNS integer AS '
DECLARE
low_number ALIAS FOR $1;
high_number ALIAS FOR $2;
result INTEGER = low_number;
BEGIN
WHILE result != high_number
LOOP result := result + 1;
END LOOP;
RETURN result;
END;
' LANGUAGE 'plpgsql';
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39. For FOR identifier IN [ REVERSE ] expression1 .. expression2 LOOP
statement;
[...]
END LOOP;
The FOR loop will perform a single iteration for each incremented value of identifier which is in the range of values between, and including, expression1 and expression2.
The identifier value will be initialized to the value of expression1, regardless of any prior settings, and incremented by one each iteration. If REVERSE is specified, identifier will be decremented rather than incremented
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40. For FOR i IN 1..10 LOOP -- some computations here END LOOP;
FOR i IN REVERSE 10..1
LOOP -- some computations here
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41. Looping Through Query Results
Method 1
FOR record_or_row IN query
LOOP statements
END LOOP;
Method 2
FOR record_or_row IN EXECUTE text_expression
END LOOP
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42. Example CREATE TABLE books ( subject_id integer, title text );
insert into books values(1, 'Book A');
insert into books values(1, 'Tomorrow');
insert into books values(2, 'Book B');
insert into books values(3, 'Book C');
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43. Looping Through Query Results
CREATE FUNCTION extract_title (integer) RETURNS text AS '
DECLARE
sub_id ALIAS FOR $1;
text_output TEXT :='' '';
row_data books%ROWTYPE;
BEGIN -- Iterate through the results of a query.
FOR row_data IN SELECT * FROM books
WHERE subject_id = sub_id ORDER BY title
LOOP
-- Insert the title of a matching book into the text_output variable.
text_output := text_output || row_data.title || '' '';
END LOOP;
RETURN text_output;
END;
' LANGUAGE 'plpgsql';
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