Application Programming Database lectures for mathematics students

Zbigniew Jurkiewicz, Institute of Informatics UW

April 23, 2017

Zbigniew Jurkiewicz, Institute of Informatics UW Application Programming Database lectures for mathematics students Functions

You can define functions in Postgres with

CREATE FUNCTION name(parameter type, ...) RETURNS result-type AS $$ function-body $$ LANGUAGE language-name;

For the SQL language the body is a sequence of SQL statements. The function body in older versions was surrounded by single quotes, which was inconvenient, as it lead to doubling any apostrophes inside body (around strings etc.).

Zbigniew Jurkiewicz, Institute of Informatics UW Application Programming Database lectures for mathematics students Functions

Because in Postgres the statement SELECT can be used for evaluating expressions not connected with any database, e.g.

bd> SELECT 2 + 5 AS seven; seven ------7

so simple functions can be defined using just SQL:

CREATE FUNCTION add7(i int4) RETURNS int4 AS $$ SELECT i + 7; $$ LANGUAGE sql;

Zbigniew Jurkiewicz, Institute of Informatics UW Application Programming Database lectures for mathematics students Functions

Example call of the above function:

bd> SELECT add7(4) AS result; result ----- 11

Zbigniew Jurkiewicz, Institute of Informatics UW Application Programming Database lectures for mathematics students Functions

Functions written in SQL return as their value the result from the last statement in the body, this is usually SELECT. If the result is not to be interpreted as a set, the first resulting row will be taken (so take care about possible ORDER BY ;-). If the query result is empty, NULL is returned.

Zbigniew Jurkiewicz, Institute of Informatics UW Application Programming Database lectures for mathematics students Functions

If an SQL function does not return anything reasonable, the result type should be declared as void, and the body may not terminate with SELECT query.

CREATE FUNCTION clean () RETURNS void AS $$ DELETE FROM Animals WHERE weight <= 0; $$ LANGUAGE SQL;

SELECT clean();

Zbigniew Jurkiewicz, Institute of Informatics UW Application Programming Database lectures for mathematics students PL/pgSQL

Usually the stored database procedures are not defined in SQL, but in procedural languages. In Postgres the default language for database server is PL/pgSQL, similar to PL/SQL from Oracle. The standard languages provided in distribution are installed using the shell script createlang. To install PL/pgSQL in the database template1 it is enough to type createlang plpgsql template1 Removal of a procedural language could be done within SQL using the statement DROP LANGUAGE or by running the script droplang.

Zbigniew Jurkiewicz, Institute of Informatics UW Application Programming Database lectures for mathematics students PL/pgSQL

PL/pgSQL (Procedural Language/postgresSQL) extends SQL with typical constructs encountered in imperative programming languages. The basic unit in PL/SQL is the block, programs are built from nested blocks. Inside blocks you may use as instructions all SQL data manipulation statement (e.g. SELECT, INSERT, UPDATE, DELETE) and transaction control statements. There is also an extended form of SELECT statement, to move the values found in the database into PL/pgSQL variables. Schema data definition instructions, such as CREATE, DROP or ALTER, are not allowed.

Zbigniew Jurkiewicz, Institute of Informatics UW Application Programming Database lectures for mathematics students PL/pgSQL

In addition to SQL statements blocks may contain typical instructions like assignments, conditional instructions, loops or procedure calls. PL/pgSQL blocks are usually placed in function bodies, and this functions can be called by using ordinary SELECT for example from psql program.

Zbigniew Jurkiewicz, Institute of Informatics UW Application Programming Database lectures for mathematics students Blocks

The syntax of a PL/pgSQL block is as follows:

[DECLARE declarations of variables, constants and local procedures] BEGIN instructions END;

(the square brackets mark the optional part, they are not syntax elements).

Zbigniew Jurkiewicz, Institute of Informatics UW Application Programming Database lectures for mathematics students Blocks

Example block:

DECLARE a NUMERIC(5); BEGIN SELECT COUNT(*) INTO a FROM EMP WHERE ENAME LIKE ’A%’; IF a > 0 THEN INSERT INTO TEMP VALUES(a); END IF; RAISE NOTICE ’OK’; END;

Zbigniew Jurkiewicz, Institute of Informatics UW Application Programming Database lectures for mathematics students Triggers

Triggers are procedures “automagically” called when some event happens, e.g. when inserting a new row into a particular table. Triggers originally were not intended to serve as a mechanism for ensuring the legality of database states (this role was to be played by integrity constraints and assertions), but to ensure the legality of transitions between states. But since no popular DBMS implement assertions (it is not easy, by the way), triggers are most often used to realize complex constraints.

Zbigniew Jurkiewicz, Institute of Informatics UW Application Programming Database lectures for mathematics students Triggers

Trigger definition contains three components Event: modification of some table, e.g. “inserting into Species table”. Condition: Boolean expression in SQL syntax. Actions: instructions to be executed, most often written in SQL or PL/SQL.

Zbigniew Jurkiewicz, Institute of Informatics UW Application Programming Database lectures for mathematics students Syntax of triggers

According to SQL standard, the trigger syntax is as follows:

CREATE [OR REPLACE] TRIGGER name {BEFORE | AFTER} INSERT OR DELETE OR UPDATE ON table FOR EACH {ROW | STATEMENT} BEGIN instruction ... END;

Zbigniew Jurkiewicz, Institute of Informatics UW Application Programming Database lectures for mathematics students Example

Let’s try to write a trigger which after removing a row from Species table would set NULLs in the species column for corresponding rows of Animals table. We can of course achieve the same effect more easily just using foreign key constraints — but it may be interesting to see how they can be implemented.

Zbigniew Jurkiewicz, Institute of Informatics UW Application Programming Database lectures for mathematics students Example

CREATE TRIGGER DelSpec AFTER DELETE ON Species FOR EACH ROW EXECUTE UPDATE Animals SET species = NULL WHERE species = OLD.species;

Zbigniew Jurkiewicz, Institute of Informatics UW Application Programming Database lectures for mathematics students Other example

Let us assume that for each species in the table Species we want to store (in a separate column) the number of animals of this kind. We start by adding the appropriate column ALTER Species ADD howmany INTEGER DEFAULT 0 CHECK (howmany >= 0);

Zbigniew Jurkiewicz, Institute of Informatics UW Application Programming Database lectures for mathematics students Other example

Time to define triggers CREATE TRIGGER InsAnimals AFTER INSERT ON Animals FOR EACH ROW EXECUTE UPDATE Species SET howmany = howmany + 1 WHERE name = NEW.species;

CREATE TRIGGER DelAnimals AFTER DELETE ON Animals FOR EACH ROW EXECUTE UPDATE Species SET howmany = howmany - 1 WHERE name = OLD.species;

Zbigniew Jurkiewicz, Institute of Informatics UW Application Programming Database lectures for mathematics students Triggers in Postgres

Triggers in PL/pgSQL always contain as their bodies a call to a previously defined procedure. The procedure should be a function with no arguments and should declare that it returns the special type TRIGGER The trigger function receives data not from ordinary functional parameters, but through the structure TriggerData. Trigger procedures should not be called directly (by name). They are called implicitly by trigger, whenever the associated events occur. The event can be one of SQL operations INSERT, DELETE or UPDATE.

Zbigniew Jurkiewicz, Institute of Informatics UW Application Programming Database lectures for mathematics students Triggers in Postgres

For each trigger you must define whether its procedure will be called BEFORE or AFTER the operation proper. Also each trigger should have declared one of two levels: row or statement. The option FOR EACH ROW declares row-level trigger, by default the trigger is statement-level. Statement-level trigger is fired only once for the whole SQL statement, but row-level trigger is independently fired once for each row being modified.

Zbigniew Jurkiewicz, Institute of Informatics UW Application Programming Database lectures for mathematics students Triggers in Postgres

The trigger definition in PL/pgSQL has the following syntax:

CREATE TRIGGER name BEFORE | AFTER INSERT | DELETE | UPDATE [OR ...] ON table [FOR EACH ROW | STATEMENT] EXECUTE PROCEDURE procedure-name(arguments);

Zbigniew Jurkiewicz, Institute of Informatics UW Application Programming Database lectures for mathematics students Triggers in Postgres

Additional remarks: The trigger function must be defined before creating trigger. The name of a trigger must be unique for a given table and is used when removing the trigger. For a single trigger one can state up to three kinds of events (INSERT, DELETE or UPDATE) activating the trigger (use OR connective between them). Examples: ... INSERT ON R ...... INSERT OR DELETE OR UPDATE ON R ...

Zbigniew Jurkiewicz, Institute of Informatics UW Application Programming Database lectures for mathematics students Triggers in Postgres

When removing a trigger it is necessary to give the table name in addition to the trigger name. DROP TRIGGER name ON table; In functions associated with row-level triggers there are two predeclared record variables in function bodies: NEW and OLD, they have as values old and new contents of the row. The trigger procedure for insertion and modification should return the appropriate record, usually it is just NEW. It can also return NULL. For triggers of the type BEFORE this means that the execution failed and the operation should be silently ignored.

Zbigniew Jurkiewicz, Institute of Informatics UW Application Programming Database lectures for mathematics students Triggers in Postgres

The triggers of the same type are run in the alphabetic order according to their names. If a trigger procedure uses SQL data modification statements, they can cause firing other triggers. This situation is called trigger cascading. There is no limit to the number of levels for cascading triggers, in particular the recursive calls are possible. The programmer is fully responsible for avoiding an infinite recursion.

Zbigniew Jurkiewicz, Institute of Informatics UW Application Programming Database lectures for mathematics students Triggers in Postgres

Time for examples. Assume we have the table

CREATE TABLE Species ( name VARCHAR(30) PRIMARY KEY, continent VARCHAR(11), preserved BOOLEAN, food VARCHAR(15) );

Zbigniew Jurkiewicz, Institute of Informatics UW Application Programming Database lectures for mathematics students Triggers in Postgres

Using trigger we will ensure that the name of the continent will always start with a capital letter:

CREATE FUNCTION normcont () RETURNS TRIGGER AS $$ BEGIN IF NEW.continent IS NOT NULL THEN NEW.continent := lower(NEW.continent); NEW.continent := initcap(NEW.continent); END IF; RETURN NEW; END; $$ LANGUAGE ’plpgsql’;

CREATE TRIGGER spectrig1 BEFORE INSERT OR UPDATE ON Species FOR EACH ROW EXECUTE PROCEDURE normcont();

Zbigniew Jurkiewicz, Institute of Informatics UW Application Programming Database lectures for mathematics students Triggers in Postgres

The execution of the statement CREATE TRIGGER only creates a trigger, but does not fire it. To fire a trigger we must cause an occurence of the appropriate event, in this case row insertion or modification for the table Species. UPDATE Species SET continent=’africa’ WHERE name=’hen’;

Zbigniew Jurkiewicz, Institute of Informatics UW Application Programming Database lectures for mathematics students Application programming

The ordinary users access the database by running programs, executed on the user’s workstation (client programs). Such programs can be written in any programming language, but the language implementation must provide a programmer interface for SQL (called sometimes API — Application Programmer Interface). This interface is sometimes called binding and could be specific for a given DBMS or universal (like ODBC, JDBC), i.e. covering any SQL database.

Zbigniew Jurkiewicz, Institute of Informatics UW Application Programming Database lectures for mathematics students Interface for C programming language

Implementors of a DBMS nearly always provide a basic procedural interface for C programming language. The SQL standard calls such type of interface CLI or Call-Level Interface Let us look briefly at the PostgreSQL interface for C language, contained in the library libpq. The application should first attach the header file: #include "libpq-fe.h"

Zbigniew Jurkiewicz, Institute of Informatics UW Application Programming Database lectures for mathematics students Interface for C programming language

Before using the database the connection has to be established: PGConn *connection;

connection = PQconnectdb("dbname=bd,host=labdb,user=me"); if (PQstatus(connection) == CONNECTION_BAD) fprintf(stderr, "No connection\n"); PQfinish(connection); exit(1);

Assume the connection succeeded and we want now to look into a table Species, which contains columns name and continent.

Zbigniew Jurkiewicz, Institute of Informatics UW Application Programming Database lectures for mathematics students Interface for C programming language

We want to find out a continent, which is the origin of the species with the given name (e.g. the name have been read from a keybord).

PGresult *result;

result = PQexec(connection, "SELECT continent FROM Species " "WHERE name = ’racoon’"); if (PQresultStatus(result) == PGRES_TUPLES_OK && PQntuples(result) == 1) printf("Racoon comes from %s\n", PQgetvalue(result, 0, 0)); else fprintf(stderr, "No answer\n"); PQfinish(connection);

Zbigniew Jurkiewicz, Institute of Informatics UW Application Programming Database lectures for mathematics students Embedded SQL

The embedded SQL is the technique of directly entering SQL statements as instructions of the host language (in our case the C language). In PostgreSQL we can use the tool called ECPG. When embedding SQL into other programming languages there are many problems to be solved, the most important being Different data types in SQL than in the embedding language — conversion is necessary. SQL is oriented towards operating on sets of values, not with single values — usual solution is to use cursors.

Zbigniew Jurkiewicz, Institute of Informatics UW Application Programming Database lectures for mathematics students Embedded SQL

In PostgreSQL the program in C with embedded SQL statements must be first preprocessed with ecpg into a program in pure C. The preprocessor finds the SQL statements contained in program and replaces them by appropriate function calls from CLI libraries. A preprocessed program is then compiled using the ordinary C compiler to obtain the executable binary.

ecpg -I/usr/include/ecpg test1.pgc cc -I/usr/include/ecpg -o test1 test1.c -lecpg

Zbigniew Jurkiewicz, Institute of Informatics UW Application Programming Database lectures for mathematics students Embedded SQL

All SQL statements must be preceded by EXEC SQL and terminated with semicolon (“;”). They can be inserted anywhere in C program, ensuring only that all declarations precede executable statements. An example: int main () { EXEC SQL BEGIN DECLARE SECTION; int w; EXEC SQL END DECLARE SECTION; ... EXEC SQL SELECT age INTO :w FROM Animals WHERE name=’Kropka’; ... printf("Kropka weights %d kilo\n", w); ... }

Zbigniew Jurkiewicz, Institute of Informatics UW Application Programming Database lectures for mathematics students