SQL in higher level languages
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SQL in higher level languages

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SQL in higher level languages SQL in higher level languages Presentation Transcript

  •  
    • SQL in higher level languages
  •   SQL in higher level languages
    • Why do we want to use SQL in a higher level language?
    • Read in data from file, insert into relation
    • Loop until the EOF
    • read values from file – higher level code
    • manipulate values with higher level code
    • insert into relation values (SQL)
    • End loop
  • Why cont’d
    • Compute results based on result from query e.g. generate a report
      • Query database
      • Compute results from query
      • Print results
    • Provide a user interface for SQL if the current one is lacking
      • Prompt user for query
      • Send query to DBMS
      • Receive results
      • Display results to user
  • Must have:
      • to do 1) must read in values into C variables then use those values to insert using SQL
      • still need SQL statement to insert, select tuples
      • to do 2) must be able to manipulate results from SQL query, but mismatch between C and SQL
      • sets versus one record at a time
      • to do 3) need to accept queries from user - create SQL queries
        • Given the query:
        • Select dnumber
        • From department
        • Where mgrssn = 987654321
    • What is needed?
      • Variables in which to place result (Host variables)
      • Processing of result table (cursors)
      • Data structure for communicating with DBS in case of errors (SQLCA)
      • What if we want to process any query typed in? (Dynamic SQL)
  • To do this
    • Can use:
      • embedded SQL
    • Precede each statement with EXEC SQL
    • 2. Platform specific classes, interfaces
    • Oracle’s OLE
    • 3. Platform independent classes, interfaces
    • JDBC
  • Host variables :
    • Referenced by SQL and higher level language
    • Prefixed with : in SQL statements
    • transmit data between DB manager and application
    • Allows value of host variable to change if SQL executed > 1
      • select ssn
      • from employee
      • where salary < :min
      • insert into project values (:projectName, :projectNumber, :projectLocation, :departmentNumber)
  • Parameter markers ?
    • Can pass query as character string (dynamic SQL)
    • “ update employee
    • set salary = salary*1.1
    • where dno = ? and bdate = ?”
    • We need to identify the variable that we will obtain later
    • Depending on system, use ? or : as a parameter marker
    • An SQL statement can contain more than one parameter marker
  • Cursors 
    • How to access multiple rows from a query result?
    •    Use a cursor 
      • A cursor points to 1 row
      • Can move forward, backwards, etc.
  • Using cursors
    • 3 steps involved:
      •   1) declare cursor - just a definition of the select
      • 2) open cursor - executes select, builds result table
        • Declare/open can be as one, e.g. OLE
      • 3) fetch results - to navigate through the results
  • SQLCA
    • SQL communication area - a structure
    • used for communication between DBS monitor and C++ program
    • allocates program space for errors and starts communication by DBS monitor
    • after each SQL statement executed, a new value is placed in SQLCA
    • indicates if successful, EOF, etc.
    • error or warning conditions
  • SQLCA
    • sqlca.sqlcode - testing code part of structure
      • sqlcode = 0 successful sql call
      • < 0 error
      • > 0 warning - call successful but some condition existed
    • e.g. EOF is 100 (DB2, Ingres, but not ORACLE)
    • e.g. in Embedded SQL, sqlerrd[2] - indicates number of row affected by insert, update or delete (used for referential integrity)
  • Error messages
    • printing error messages - can extract the error message
    • Available through SQLCA
    •  
  • Updating
    • Delete
      • Delete tuples from base table
      • Positioned delete, use cursor
    • Updates
      • Makes changes to base table
      • Positioned delete, use cursor
    • Insert
      • No need for cursor, can't specify position of new row
    • DDL
      • Can create tables, etc.
  • Dynamic SQL
    • Dynamic SQL
      • Useful when:
      • Format of SQL statement is not known - can generate during execution
      • Statement known but objects referenced don't exist at compile time
    • Used by OLE, JDBC, etc.
  • OLE, JDBC
    • OLE methods are available in Oracle to do most of the above
    • Can also use JDBC, independent of platform