Lecture 26 10/28/15. CMPSC431W: Database Management Systems. Instructor: Yu- San Lin

Transcription

1 CMPSC431W: Database Management Systems Lecture 26 10/28/15 Instructor: Yu- San Lin Course Website: hcp:// Slides based on McGraw- Hill & Dr. Wang- Chien Lee 1

2 CHAPTER 6: DATABASE APPLICATION DEVELOPMENT 2

3 Overview SQL in applicaton code Connect to a DBMS Manipulate data retrieved from a DBMS Modify data in DBMS Embedded SQL Cursors Dynamic SQL JDBC SQLJ Stored procedures 3

4 SQL in ApplicaTon Code SQL commands can be called from within a host language program, e.g., C++, Java, Python SQL statements can refer to host variables (including special variables used to return status) Must include a statement to connect to the right DB 4

5 SQL in ApplicaTon Code (cont.) Two main integraton approaches: Embed SQL in the host language, e.g., Embedded SQL, SQLJ Create special to call SQL commands, e.g., JDBC Impedance mismatch: SQL relatons are (mult- ) sets of records, with no a priori bound on the number of records SQL supports cursor to handle mismatch between SQL & programming language 5

6 Embedded SQL Embed SQL in the host language (C in this chapter s discussion) A preprocessor converts the SQL statements into special API calls Then a regular compiler is used to compile the code Language constructs: ConnecTng to a database EXEC SQL CONNECT Declaring variables EXEC SQL BEGIN (END) DECLARE SECTION Statements EXEC SQL Statement 6

7 Embedded SQL: Variables EXEC SQL BEGIN DECLARE SECTION char c_sname[20]; long c_sid; short c_rating; float c_age; EXEC SQL END DECLARE SECTION Two special error variables: long SQLCODE is negatve if an error has occurred char[6] SQLSTATE predefined codes for common errors 7

8 Cursors Declare a cursor on a relaton or query statement (which generates a relaton) A cursor allows us to retrieve the rows in a table by positoning the cursor to a partcular row and reading its contents Open a cursor; repeatedly fetch a tuple and move the cursor untl all tuples have been retrieved 8

9 Cursors (cont.) The cursor can move to partcular rows by specifying additonal parameters for the FETCH command Queries accessed through a cursor can control the order of tuples returned by using a ORDER BY clause Fields in ORDER BY clause must also appear in SELECT clause The ORDER BY clause, which orders answer tuples, is only allowed in the context of a cursor Can also modify/delete tuple pointed to by a cursor 9

10 Get names of sailors who have reserved a red boat, in alphabetcal order EXEC SQL DECLARE sinfo CURSOR FOR SELECT S.sname FROM WHERE Sailors S, Boats B, Reserves R S.sid = R.sid AND R.bid = B.bid AND B.color = red ORDER BY S.sname * Note that it is illegal to replace S.sname by S.sid in the ORDER BY clause 10

11 Cursors: More Details We usually need cursor for a SELECT and do not need it for INSERT, DELETE, and UPDATE. Why? Answer: 11

12 Embedding SQL in C: Example char SQLSTATE[6]; EXEC SQL BEGIN DECLARE SECTION char c_sname[20]; short c_minrating; float c_age; EXEC SQL END DECLARE SECTION c_minrating = random(); EXEC SQL DECLARE sinfo CURSOR FOR SELECT S.sname, S.age FROM Sailors S WHERE S.rating > :c_minrating ORDER BY S.sname; do { EXEC SQL FETCH sinfo INTO :c_sname, :c_age; printf( %s is %d years old\n, c_sname, c_age); } while (SQLSTATE!= ); /* there are more rows */ EXEC SQL CLOSE sinfo; 12

13 Dynamic SQL SQL query strings are not always known at compile Tme. This allows constructon of SQL statements to be on- the- fly Example: char c_sqlstring[]= { DELETE FROM Sailors WHERE raiting>5 }; EXEC SQL PREPARE readytogo FROM :c_sqlstring; EXEC SQL EXECUTE readytogo; The preparaton of Dynamic SQL command occurs in runtme, which incurs 13

14 DB APIs: AlternaTve to Embedding A problem with embedded SQL: Even though the source code can be compiled to work with different DBMSs, the final executable usually only works with one No standard for accessing different DBMSs We say Embedding SQL is 14

15 DB APIs: AlternaTve to Embedding (cont.) Rather than modifying compiler, introducing a layer of abstracton between applicaton and DBMS Add drivers with database calls Special standardized interface: procedures/objects Pass SQL strings from language, presents result sets in a language- friendly way Examples: Open DataBase ConnecTvity (ODBC), Java DataBase ConnecTvity (JDBC) We say this the API approach is 15

16 How DB API Approach Works An applicaton interacts with a data source Select a Dynamically load the corresponding Establish a with the data source Translate/transmit the SQL statements Execute the translated SQL statements Return the Note: No limit on the number of open connectons An applicaton can have several open connectons to different data sources 16

17 JDBC: Architecture IniTate and terminates connectons Sets transacton boundaries Submits SQL statements Receives results Load JDBC driver Pass (and log) JDBC functon calls to the correct drivers 17

18 JDBC: Architecture (cont.) Establish connectons to data source Transmits requests and returns results and error codes Translate results and error codes from a data source natve form into JDBC standard Processes commands from the driver and returns results 18

19 Drivers in JDBC Depending on the relatve locaton of the and the, several architectural scenarios are possible Drivers in JDBC are classified into four types: Bridges Direct translaton to natve API via non- Java driver Network bridges Direct translaton to the natve API via Java driver 19

20 Type I: Bridge Translates JDBC functon calls into functon calls of another API that is not natve to the DBMS Example: JDBC- ODBC bridge allows JDBC applicaton to access an ODBC compliant data source Advantage: easy to piggy- back the applicaton onto an existng installaton Disadvantages: Increased number of layers between data source and applicaton affects performance User is limited to the functonality ODBC driver supports 20

21 Type II: Direct TranslaTon to the NaTve API via Non- Java Driver Translates JDBC functon calls directly into method invocatons of the API of one specific data source Advantage: performs significantly becer than bridge because it s wricen using a combinaton of C++ and Java Disadvantage: database driver that implements the API needs to be installed on each computer that runs the applicaton 21

22 Type III: Network Bridges The driver talks over a network to a middleware server that translates the JDBC requests into DBMS- specific method invocatons Driver on the client side DBMS- specific Needs only small JDBC driver at each client 22

23 Type IV: Direct TranslaTon to the NaTve API via Java Driver The driver communicates with the DBMS through Driver on the client side is wricen in Java, DBMS- specific It translates JDBC calls into the natve API of the database system 23