SQL is capable in manipulating relational data SQL is not good for many other tasks

Transcription

1 Embedded SQL

2 SQL Is Not for All SQL is capable in manipulating relational data SQL is not good for many other tasks Control structures: loops, conditional branches, Advanced data structures: trees, arrays, Not good for general problem solving How to solve a general problem involving relational data? Use a general programming language (e.g., C++/Java) in general but use SQL for relational data access How to embed SQL into a general programming language? CMPT 354: Database I -- Embedded SQL 2

3 Integrating SQL and Programming? Typical procedural programming languages C/C++, Java Programs = data structures + algorithms Structured data variables: records, arrays Procedural semantics: loop, control structures SQL Sets: query results are sets, unordered in general No control structure in general Hiding data structures and algorithms CMPT 354: Database I -- Embedded SQL 3

4 Programming Environment Database management system program? SQL files database storage CMPT 354: Database I -- Embedded SQL 4

5 SQL and Hosting Language Database management system program embedded SQL files database storage CMPT 354: Database I -- Embedded SQL 5

6 Embedding SQL Host language + Embedded SQL Host language + Function calls preprocessor Host-language compiler SQL library Pre-compile Object-code program CMPT 354: Database I -- Embedded SQL 6

7 SQL/Host Language Statement? How can a preprocessor tell whether a statement is for SQL or for the host language? Keywords EXEC SQL in front of the statement How can an SQL statement use a variable in the host language? Insert a new customer whose information is input by a user char customer_name[20], customer_street[30], customer_city[30], SQLSTATE[6]; //host language stuff EXEC SQL BEGIN DECLARE SECTION; char customer_name[20], customer_street[30], customer_city[30]; char SQLSTATE[6]; EXEC SQL END DECLARE SECTION; Declare a host variable in SQL before using it in SQL statements CMPT 354: Database I -- Embedded SQL 7

8 Embedded SQL SQL can be embedded in many host languages such as C/C++, Java, and COBOL EXEC SQL statement is used to identify embedded SQL request to the preprocessor The basic form follows that of the System R embedding of SQL into PL/I EXEC SQL <embedded SQL statement> END_EXEC May vary by language (for example, the Java embedding uses # SQL {. }; ) CMPT 354: Database I -- Embedded SQL 8

9 Using Shared Variables EXEC SQL BEGIN DECLARE SECTION; char customer_name[20], customer_street[30], customer_city[30]; char SQLSTATE[6]; EXEC SQL END DECLARE SECTION; customer_name=get_customer_name(); customer_street=get_customer_street(); customer_city=get_customer_city(); EXEC SQL INSERT INTO customer(customer_name, customer_street, customer_city) VALUES (:customer_name, :customer_street, :customer_city); The shared variables are prefixed by a colon in SQL statements They appear without the colon in host language statements CMPT 354: Database I -- Embedded SQL 9

10 Status of SQL Statement Execution How can we know whether an update succeeds or not? SQLSTATE connects the host language program with the SQL execution system SQLSTATE: an array of five characters Each time a function of the SQL library is called, a code is put in SQLSTATE : no error condition occurred : a tuple requested is not found CMPT 354: Database I -- Embedded SQL 10

11 Communication in Between Database management system program Data SQLSTATE SQL files database storage CMPT 354: Database I -- Embedded SQL 11

12 How to Use SQLSTATE An easy way is to treat it as a string Special considerations for host languages, e.g., C/C++ A string in C/C++ needs an end-of-string character \0 Declare char SQLSTATE[6]; Set SQLSTATE[5]= \0 ; Use strcmp in comparison CMPT 354: Database I -- Embedded SQL 12

13 Storing Results into Variables If an SQL statement returns only one tuple, the attribute values can be stored into variables in the host language Use keyword INTO in the SELECT clause EXEC SQL SELECT SUM(assets) INTO :total_assets FROM branch WHERE branch_city = Brooklyn ; CMPT 354: Database I -- Embedded SQL 13

14 Handling a Set of Tuples Generally, an SQL statement may return a set of tuples Cursor Conceptually, put all tuples returned by a query into a file Fetch the tuples from the virtual file After use, the file should be closed and released CMPT 354: Database I -- Embedded SQL 14

15 Example From within a host language, find the names and cities of customers with more than the variable amount dollars in some account EXEC SQL declare c cursor for select customer_name, customer_city from depositor, customer, account where depositor.customer_name = customer.customer_name and depositor account_number = account.account_number and account.balance > :amount END_EXEC Declared, but not materialized yet CMPT 354: Database I -- Embedded SQL 15

16 Executing a Cursor The open statement causes the query to be evaluated EXEC SQL open c END_EXEC The fetch statement causes the values of one tuple in the query result to be placed on the host language variables EXEC SQL fetch c into :cn, :cc END_EXEC Can be called repeatedly to get all tuples CMPT 354: Database I -- Embedded SQL 16

17 Executing a Cursor (2) SQLSTATE set to to indicate no more data is available The close statement causes the database system to delete the temporary relation that holds the result of the query EXEC SQL close c END_EXEC Details vary with language The Java embedding defines Java iterators to step through result tuples CMPT 354: Database I -- Embedded SQL 17

18 Updates Through Cursors To update tuples fetched by cursor, declare the cursor for update declare c cursor for select * from account where branch_name = Perryridge for update To update tuple at the current balance of cursor c update account set balance = balance where current of c CMPT 354: Database I -- Embedded SQL 18

19 Concurrent Changes Two users may change tuples in a database concurrently Process 1: remove account of balance < 1000 Process 2: increase the balance by 10% Whether an account of balance 950 would be removed depends on which process touches the tuple first Keyword INSENSITIVE makes a cursor not affected by other concurrent updates EXEC SQL DECLARE accountcursor INSENSITIVE CURSOR FOR SELECT balance FROM account; More about concurrency control in CMPT 454 CMPT 354: Database I -- Embedded SQL 19

20 READ ONY Cursors If a cursor is used only for read operations, it can be concurrent with any other processes EXEC SQL DECLARE readacccursor CURSOR READ_ONLY FOR SELECT balance FROM account; Different database systems may have slightly different syntax CMPT 354: Database I -- Embedded SQL 20

21 Scrolling Cursors Random access versus serial access Serial access: fetch the next one, cannot jump to other tuples Random access: fetch any tuple in a cursor Scrolling cursor: a cursor for random access EXEC SQL DECLARE acccursor SCROLL CURSOR FOR account; Options in FECTCH NEXT or PRIOR FIRST or LAST RELATIVE k (RELATIVE -1 = PRIOR) ABSOLUTE k (ABSOLUTE -1 = LAST) CMPT 354: Database I -- Embedded SQL 21

22 Dynamic SQL Allow programs to construct and submit SQL queries at run time char * sqlprog = update account set balance = balance * 1.05 where account_number =? EXEC SQL prepare dynprog from :sqlprog; char account [10] = A-101 ; EXEC SQL execute dynprog using :account;? is a place holder for a value that is provided when the SQL program is executed CMPT 354: Database I -- Embedded SQL 22

23 Summary SQL is good for accessing relational data, but not for other general problem solving tasks Embedding SQL into a host general programming language is useful in practice Embedded SQL Cursors CMPT 354: Database I -- Embedded SQL 23

24 To-Do-List Understand how to embed SQL into C# Understand cursors in SQL Server 2005 CMPT 354: Database I -- Embedded SQL 24