13 Creation and Manipulation of Tables and Databases

Transcription

1 Informationslogistik SQL Handout No. 9 SS Creation and Manipulation of Tables and Databases 13.1 Creation and Deletion Databases can be created and deleted using respectively. CREATE DATABASE dbname and DROP DATABASE dbname Tables are created according to the following syntax. [] indicate optional arguments. CREATE TABLE tabname( col1 type[(length1)] [NOT NULL] [DEFAULT default1] [AUTO INCREMENT], col2 type[(length2)] [NOT NULL] [DEFAULT default2] [AUTO INCREMENT],..., [PRIMARY KEY (col1a,col1b,...),] [FOREIGN KEY (col2a,col2b,...) REFERENCES tab (tabcol1,tabcol2,...)] ) The type indicates the data type of the respective column. Common data types are int, float, double, date and varchar. For some data types (e.g. varchar) the length option is obligatory. With NOT NULL you indicate that it is forbidden that the value shall be NULL, while with DEFAULT you can define a default value. AUTO INCREMENT is another useful option for creating columns that shall contain e.g. consecutive numbers. By the way, you can take a look at the statements that were used to create a table at the bottom of the Objects-tab in SQLYog. In order to drop a table use DROP TABLE tabname Example. We would like to create two tables containing information about students and their branch of study. We create a table students with columns matrnr, name and study. Another table studies shall contain the Studienkannzahl stkz and the name of the branch of study (column branch). Obviously, each stkz identifies the branch so that it will serve as primary key for studies. In students our primary key consists of matrnr and study as each student may study more than one branch, so that matrnr identifies the name of a student but not necessarily a whole row in the table students. Finally, there is the foreign key study in students that refers to stkz in studies. Thus, these two tables are created by CREATE TABLE studies ( CREATE TABLE students ( stkz varchar(10) NOT NULL, matrnr varchar(7) NOT NULL, branch varchar(50) NOT NULL, name varchar(50) NOT NULL, PRIMARY KEY (stkz) study varchar(10) NOT NULL DEFAULT , ) PRIMARY KEY (matrnr, study), FOREIGN KEY (study) REFERENCES studies(stkz) ) Note that it is important to create studies first as otherwise it is not possible to refer to its primary key when creating students.

2 13.2 Insert and Change of Values in(to) Tables Inserts can be performed either using to insert single rows or INSERT INTO tabname (col1, col2,...) VALUES (val1, val2,... ) INSERT INTO tabname (col1, col2,...) SELECT... to insert the result of a query into the table. It is not necessary to insert values for all columns. Columns not mentioned usually get default values (if defined when the table was created, see above) or are filled up with NULL. If the column was defined AUTO INCREMENT a consecutive number is created. Example. We insert the branch Industrielogistik into branch. INSERT INTO studies (stkz, branch) VALUES ( , Industrielogistik Bakk. ) Then we insert all MUL students from the table stud of the Students database into students. INSERT INTO students (matrnr, name) SELECT mn, CONCAT(vorname,, nachname) FROM oracle1 stud.stud WHERE SUBSTR(mn,3,2) = 35 Note that as default study for all students is inserted. Deletion of rows can be accomplished by statements of the form DELETE FROM tabname WHERE condition Example. We delete from students all students who registered in DELETE FROM students WHERE LEFT(matrnr,2) = 96 Addition and Deletion of columns is achieved by ALTER TABLE tabname ADD COLUMN col type[(length)] ALTER TABLE DROP COLUMN col Updates of values are performed with UPDATE tabname SET col1 = value1, col2 = value2,...

3 It is possible to use correlated subqueries to update. Example. We want to replace the column name with two columns containing first name and surname. First we add the two new columns. ADD COLUMN first name varchar(25) ADD COLUMN surname varchar(35) In order to get the correct first and surnames, we can either split the values in name to update the new columns. UPDATE students SET surname = SUBSTR( name, INSTR(name, ) ), first name = LEFT( name, INSTR(name, ) - 1 ) The other way would be to use a correlated subquery on the original table stud. Here is the statement to fill the column surname. Finally, we delete the column name. UPDATE students s SET surname = ( SELECT nachname FROM oracle1 stud.stud os WHERE os.mn = s.matrnr ) DROP COLUMN name 13.3 Views Views are a mixture between a query and a table. You can create a view by CREATE VIEW viewname AS SELECT... Then you can treat the view just like a table and query with SELECT * FROM viewname. The result of such a query is the same as if you would execute the query in the definition of the view. As the query itself a view changes whenever the underlying tables change. A view can be changed using REPLACE VIEW which has the same syntax as CREATE VIEW. Example. CREATE VIEW m numbers AS SELECT DISTINCT matrnr FROM students Use SELECT * FROM m numbers to obtain all matriculation numbers. Delete some matriculation number from students and execute the query again.

4 14 Embedding SQL into Java Installation In order to be able to connect a MySQL database from Java, you just have to download the MySQL Connector jar file and make sure that your Java installation includes this file. Executing a query Here is minimal Java code for executing a query. 1 import java. sql.*; public class Select { public static void main ( String args []){ 6 try { Statement stmt ; ResultSet rs; 11 String s = " SELECT name, area FROM cia WHERE region = Europe "; // register JDBC driver for MySQL Class. forname ( " com. mysql. jdbc. Driver " ); 16 // define URL for database String url = " jdbc : mysql :// :3306/ oracle1_00_cia "; // connect to database Connection con = DriverManager. getconnection ( 21 url, " oracle00 ", " password " ); // set Statement object stmt = con. createstatement (); 26 // execute query and store result in ResultSet object rs = stmt. executequery ( s ); // read result using a loop while ( rs. next () ){ 31 String name = rs. getstring ( " name " ); int area = rs. getint ( 2 ); System. out. println ( name + " has an area of " + area + " km ^2" ); 36 // close connection con. close (); catch ( Exception e ) { System. out. println ( " Error " + e ); 41

5 Java method SQL Integer getint() INTEGER float getfloat() REAL double getdouble() FLOAT boolean getboolean() BIT String getstring() CHAR/VARCHAR Date getdate() DATE Time gettime() TIME Some explanations: The the database connection part must be embedded into a try-catch- environment (used for error handling). There are special classes like Connection, Statement and ResultSet. The class Statement is used to send a query to the database, while the result is stored into an object of the class ResultSet. In line 20 the connection to the database is established. You have to provide the usual data needed when logging into the database. Thus, the string url contains the MySQL HostAddress, the Port (separated by a colon) and the database name. The method getconnection takes as parameters the username and the password for the login. The query we want to execute is stored into a string s and executed in line 27. The result is stored into the ResultSet rs. In order to read the data from the ResultSet a loop is used (lines 30 34). Basically, the ResultSet rs contains the result table of the executed query, where you have access to a single row. The method next() checks whether there are any rows left and reads the next row in case there is one. In order to read individual column values, one uses methods like getint, getstring etc. (see table), depending on the datatype of the attribute. As parameter for these methods one may either use the column header or the number of the column in the query (cf. lines 31 and 32). At the end the connection is closed (l.37). Other methods and classes: Beside the method next(), the class ResultSet provides the methods previous(), first(), and last() for scrolling in the result table, and the method wasnull() for checking whether a read value is NULL. In order to avoid SQL injections (see e.g. ) one can use so-called prepared statements. You can find examples for download on the website. Updates, Inserts and other similar commands work without ResultSet. Instead of the method executequery(string s) one uses the method stmt.executeupdate(string s). Thus, a modified mainmethod for executing an update could look like this. public static void main ( String args []){ 2 try { // as above... 7 String s = " UPDATE cia SET population = 8E6 WHERE name = Austria "; stmt = con. createstatement (); 12 // execute update stmt. executeupdate ( s ); con. close (); 17 catch ( Exception e ) { System. out. println ( " Error " + e );