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  • 1. June 15, 2002 Volume 3 Issue 05 DB2 and You A Newsletter to give tips, answer questions and expand our knowledge definition fits the DB2 usage perfectly. What do cursors mean to DB2? When Tip of the Month you execute SQL SELECT statements, DB2 returns the entire set of rows (called the answer set) that satisfy the SELECT. A typical program would likely suffer if it Cursors! Foiled No More had to deal with the entire answer set Willie Favero 1 (which could contain thousands, if not tens of thousands, of rows) returned from They're necessary in nearly every DB2 a SELECT. All the returned rows have to program. But with their one-directional be placed somewhere; programmers abilities, cursors could seem a formidable wanted a mechanism that allowed foe. Arriving to save the day? New, programs to process all that data without scrollable cursors in DB2 UDB for z/OS bringing it into working storage. and OS/390. Enter the cursor. Cursors allow the Have you ever driven a car? Ever use program to point to a single row within reverse? Dumb question, isn't it? Driving the result, retrieve and process that row, would be a logistical nightmare if you and then move on to the next row if could only drive forward. What about necessary. But before v.7, they could using a phone book? Can you image only move on to the next row, they having to go back to the first page of the couldn't return or move backward. phone book to start your search over Before you can appreciate what an again because you passed your listing or advance this feature is, you have to wanted to read the listing again? understand the limitations imposed by the Although these examples sound far- original cursors. fetched, they illustrate the challenge that faced application developers every time IN THE BEGINNING they wrote a program that used cursors. DB2 has been around a fairly long time Fortunately, that method of programming — some 18 years. In all that time, the will soon be a distant memory, thanks to SQL provided with DB2 for navigating the scrollable cursor feature of DB2 data from an application program really Universal Database (DB2 UDB) for z/OS hasn't changed. If you have more than and OS/390 Version 7. one row to read, you use a cursor. Merriam-Webster's Collegiate Dictionary defines a cursor as "a movable pointer used to mark a position." And that INSIDE 1 Staff Development 2 PeRFect PRF 1 From DB2 Magazine 2nd Quarter, 2002 3 Fun Stuff
  • 2. For most application developers, using a cursor name itself is simply an SQL long cursor in a program is just a way of life — identifier (18 or less characters) unique sometimes a difficult way of life. As within that program. WITH HOLD and required since DB2's introduction, an WITH RETURN give the cursor special application program must first declare a characteristics. WITH HOLD prevents the cursor. Doing so amounts to defining cursor from being closed by a commit, what the SELECT statement should look thus maintaining the cursor's position. like to generate a set of rows that the WITH RETURN is used by a DECLARE program could then process. After CURSOR in a stored procedure to make declaring the cursor, the next step is to that cursor's answer set available to the open that cursor in the processing part of program calling the stored procedure. the program. The open cursor actually WITH RETURN, introduced with v.5, executes the SELECT statement, asking made stored procedures really useful. DB2 to prepare a set of rows as an The answer set for the declared cursor is answer set for additional processing. created when the cursor is opened using With the cursor opened and an answer the SQL OPEN CURSOR statement. set created, the program can fetch rows OPEN CURSOR causes DB2 to execute from the answer set. the SELECT in the DECLARE statement. But here is where the problems began. Of course, the OPEN may not always SQL FETCH statements could only fetch create an answer set; it may just allow forward, retrieving the next row from the the FETCH to retrieve rows directly from answer set and returning the row to the the base table. (If an answer set is program for processing. Any need to created, all the rows that satisfy the reread the current row or (heaven forbid) SELECT are read before the OPEN go back and get a previously read row completes. If no answer set is necessary, meant you were out of luck. If you hadn't the rows are read directly from the table saved the answer set somehow, the as needed. Performance is better when program would have to close the cursor, reading directly from the table.) Any host reopen it, and start the fetch process all variables specified in the DECLARE over. statement's SELECT would have to be initialized prior to issuing the open. WORKING WITH CURSORS The SQL FETCH CURSOR statement makes a row available to a program. Using cursors in a program requires Prior to v.7, the FETCH would always adding four SQL statements: DECLARE, retrieve the next row until no more rows OPEN, FETCH, and CLOSE CURSOR. were available. On the FETCH after The DECLARE CURSOR statement reading the last row, DB2 would return a describes how the set of rows (the +100 SQLCODE to the application to let answer set) will be created, identifies the the program know that there were no name of the cursor associated to that more rows to process. But because it answer set, and describes any special only allowed you to fetch forward, DB2 cursor characteristics. Creating an couldn't help if you needed to reread the answer set is simple. The SQL SELECT current row or a previous row (for statement coded within the DECLARE example, to make sure the value hasn't identifies the tables and rows that will be changed before updating a column). included in the answer set. The SELECT The CLOSE CURSOR statement closes can be as simple or complex as the the cursors when processing is complete. programmer wants. There are no If a close isn't issued, the cursor remains restrictions. It can be dynamic or static open until the program terminates, which SQL and can contain host variables. The could create locking implications in some programmer needs to decide if the cursor situations. will be updateable or read-only. The
  • 3. USING SCROLLABLE CURSORS declared temporary table for retrieving rows and never reuses the base table. DB2 v.7 alleviates the fetching forward limitation by implementing scrollable Only updates and deletes are reflected in cursors. Scrollable cursors give the answer set of a sensitive cursor. If an application programs the ability to scroll update or delete is processed against the forward, backward, or to a relative or table and the cursor is defined as absolute position anywhere in the answer sensitive, the change is reflected in the set. Originally written up in the SQL3 answer set as an update or delete hole. ANSI Standard and later moved into the Inserts are never reflected in the answer SQL99 Standard, the definition of how set. These holes present special scrollable cursors should work has been processing challenges for programmers: around for a number of years. Most of the They will need to add extra code to their standard's definition is included in v.7. applications to test the SQLCODE returned. This test avoids processing a In order to give the FETCH statement hole. scrolling capabilities, you add the new keyword SCROLL to the DECLARE In addition to the FETCH improvements CURSOR definition. Before you can I've described, the standard allows for change all your applications, though, you eight more methods to manipulate a need to perform some planning and prep cursor, plus the original NEXT. Here's an work. introduction to each: Scrollable cursors require declared global • NEXT, the default, should be the most temporary tables. So you need to create familiar cursor movement as it's been a temporary database specifying the AS used since DB2's beginnings. NEXT TEMP keyword. AS TEMP tells DB2 that positions the cursor on the next row only declared temporary tables can use relative to its current position in the this temporary database. Within this answer set. If there are no more rows database, tablespaces of adequate size available, a +100 is returned to the and number must be defined. These program. tablespaces are used to maintain the • PRIOR moves the cursor to the previous cursor's answer sets. Make sure you row relative to the cursor's current position create tablespaces large enough to and retrieves the row. An error occurs if it handle the answer sets your scrollable positions the cursor before the first row cursors are going to create. The declared and a warning occurs if it attempts to temporary table is only accessible by the position the cursor on a hole. cursor that created it and is dropped when the cursor is closed. The DBA or • FIRST positions the cursor on the first row programmer must define the temporary in the answer set and then retrieves that database prior to implementing scrollable row. If the first row no longer exists and cursors. In a data-sharing environment, a there is now a hole, a message is returned temp workspace must be defined to each to the application and no values are data-sharing member. assigned. After you define the temporary database • LAST positions the cursor on the last row and update the DECLARE CURSOR with in the answer set and then retrieves that the keyword SCROLL, you're ready to row. If the last row no longer exists and scroll. When specifying SCROLL, you there is now a hole, a message is returned have to specify SENSITIVE STATIC, or to the application and no values are INSENSITIVE (there is no default). If you assigned. specify SENSITIVE, changes made by • ABSOLUTE cursor positioning will move your cursor and other cursors are visible the cursor to an absolute position (a to your application. INSENSITIVE uses a specific row relative to the top or bottom of the answer set depending on the plus or
  • 4. minus number). The absolute position can a SELECT was successful and didn't just be specified by a constant or a host get a SQLCODE 0. variable. Finally, when defining the host variables • RELATIVE cursor positioning will move used for positioning on a FETCH the cursor a plus or minus number of rows RELATIVE or ABSOLUTE, you probably relative to the current position. Relative want to use integer. Although decimal position can be specified using a constant can be used, you have to be careful to or a host variable. specify a zero value for the decimal portion of the number. It's difficult for DB2 • BEFORE only positions the cursor before to move the cursor in increments of less the first row in the results table and no than one. data is returned. The CLOSE cursor statement remains • AFTER positions the cursor after the last unchanged when defining a scrollable row in the answer set and no data is cursor. However, the OPEN behaves returned. slightly differently. The OPEN will return, • CURRENT may be the most exciting of via the SQLCA, whether the cursor is all. It doesn't reposition the cursor; it scrollable or not in SQLWARN1. It will simply returns the current row again. also return if the cursor is sensitive or CURRENT is the same as FETCH insensitive static in SQLWARN5. This RELATIVE 0. latter information can be useful in applications using the open database BE VIGILANT connectivity (ODBC) driver that need to determine the sensitivity because the There are a few quirks you need to be cursor has been defined as SENSITIVE. aware of when using scrollable cursors. Also at OPEN, after all of the rows have When executing a FETCH NEXT, been moved to the temporary table, all PRIOR, or CURRENT and the cursor is locks are released from the base table positioned on an update or delete hole, unless isolation-level repeatable read or DB2 returns no values to the host read stability are specified. variables and a warning is issued to the Another v.7 feature may come into use program. The program must check for when working with scrollable cursors: this warning because the FETCH leaves FETCH FIRST n ROWS ONLY. When the cursor positioned on the hole and any added to a SELECT, this phrase allows values in the host variables are no longer only the number of rows specified by n to valid. be retrieved from the answer set. In other Here's a neat feature: When issuing a words, a fixed number of rows that FETCH LAST or AFTER or when the satisfies the WHERE predicate is application receives a +100 from the retrieved rather than the entire answer fetch, DB2 supplies the number of rows set. One use of this phrase would be to in the answer set to the program via the retrieve a single row from a table without SQLCA using the variables SQLERRD(1) using a cursor, regardless of the number and SQLERRD(2). Of course, any host of rows in that table. However, it could variables on the FETCH are again not also be used to limit the number of rows updated. In both cases, the host from the answer set that must be stored variables may contain the values from the in the temporary table. last successful FETCH. Host variables If you decide to access scrollable cursors should never be trusted if the FETCH from an application using ODBC or Java returned a nonzero SQLCODE. By database connectivity (JDBC), the externalizing the number of rows application needs to make sure that returned, an application can validate that scrolling is available at the server it is going to access. JDBC and ODBC make
  • 5. no assumptions about SQL's capabilities. KEEPing a new table If the table-name If using ODBC, the program should issue used with the KEEP command is not an the getINFO() call. If using Java, the already-existing table, it is created in the program should issue a JDBC getTYPE() default database, in its own tablespace. call to determine if scrollable cursors are All detail line data, including query and allowed. form expressions, are saved into the Right now, DB2 offers static scrolling — a table. Column names are built from the great improvement. The next step may first 18 characters of the query field be to add dynamic scrolling. What is the names, with underscores in place of any difference? Dynamic scrolling works invalid characters. directly against the table being accessed during cursor scrolling. Static scrolling KEEPing an existing table If the table only works against a declared temporary in your KEEP command already exists, all table in a temp database. previously existing data is deleted. The BEYOND THE SURFACE structure of the new data must be exactly like that of the existing table, or an SQL This article should give you some insight error will result, just as if you tried to into how scrollable cursors can make insert a row of improperly formatted data programming DB2 applications easier into a table. Your SQLID must have while allowing the programmer more insert and delete authority for the table. choices, flexibility, and power. In fact, you may find that this feature alone may Accessing the table is as simple as make it worth your while to migrate to accessing any other DB2 table. In PRF, DB2 v.7. But this is merely an overview. on the Select Tables screen, enter your For all the nitty-gritty details, see the DB2 SQLID next to ID>, as the table creator v.7 Presentation Guide (SG24-6121), name. A list of tables kept under your ID SQL Reference (SC26-9944), and will appear, and you can select the one Application Programming and SQL Guide you need. Tables created using KEEP (SC26-9933). And have fun scrolling! may also be accessed in SPUFI, prefaced by the SQLID. CCSD developers have found KEEP PeRFect PRF handy in a few scenarios where doing all the processing in one PRF was either not possible or very complex. PRF is the Platinum Report Facility, a handy tool for creating reports from DB2 Mailing lables including a literal: Often without writing a program. mailing labels require a literal line added, such as “To the parent of:”. This is easily This month, we look at the KEEP accomplished in SQL by selecting the command to save detail data from a desired literal along with the address query or report in a DB2 table, providing columns. However, when the addresses the ability to re-use the result data with are in a VSAM file it is not so easy. One other queries or at other times. CCSD developer used KEEP to quickly Using the KEEP Command After store the addresses from the VSAM file running a query or report, while the into a DB2 table, then was able to select results are on your screen, enter KEEP the literal and create the labels. table-name. A table will be created with Simplifying complex queries: Another your SQLID as the owner, containing the CCSD developer had a very complex query results you see on the screen as query requiring many CASE statements rows and columns in the new table.
  • 6. involving a substring of a column. By 6. Macintosh would make a car that was breaking down the query to first create a powered by the sun, reliable, five times as new column for the substring and fast, and twice as easy to drive, but would KEEPing the result, then using a second only run on five percent of the roads. query for the CASE statement, the 7. The oil, water temperature and developer saved the time of writing a alternator warning lights would be COBOL program to create the report. replaced by a single "general car default" warning light. 8. New seats would force everyone to Staff Development have the same size butt. 9. The airbag system would say "Are you sure?" before going off. Date Topic 10. Occasionally for no reason whatsoever, your car would lock you out 06/19/2002 Dr. Brody will and refuse to let you in until you address the group simultaneously lifted the door handle, 06/26/2002 How to play in our turned the key, and grab hold of the radio antenna. sandboxes 11. GM would require all car buyers to also purchase a deluxe set of Rand Fun Stuff McNally road maps (now a GM subsidiary), even though they neither need them nor want them. Attempting to delete this option would immediately If GM had developed technology like cause the car's performance to diminish Microsoft, we would all be driving cars by 50% or more. Moreover, GM would with the following characteristics: become a target for investigation by the 1. For no reason whatsoever your car Justice Department. would crash twice a day. 12. Every time GM introduced a new 2. Every time they repainted the lines on model car buyers would have to learn the road you would have to buy a new how to drive all over again because none car. of the controls would operate in the same manner as the old car. 3. Occasionally your car would die on the freeway for no reason, and you would just 13. You'd press the "start" button to shut accept this, restart and drive on. off the engine. 4. Occasionally, executing a maneuver such as a left turn, would cause your car to shut down and refuse to restart, in which case you would have to reinstall the engine. 5. Only one person at a time could use the car, unless you bought "Car95" or "CarNT." But then you would have to buy more seats.