Cursors are used when the user needs to update records in a singleton fashion or in a row by row manner, in a database table. Hence the size of the cursor is limited by the size of this pre-defined area. Attention reader! Fetch: When the cursor is opened, rows can be fetched from the cursor one by one or in a block to perform data manipulation. Close: After data manipulation, close the cursor explicitly. Deallocate: Finally, delete the cursor definition and release all the system resources associated with the cursor.
Define different levels of sensitivity to data changes made by other users. You can use cursors in a variety of ways: With no rows at all. With some or all of the rows in a single table.
With some or all of the rows from logically joined tables. As read-only or updateable at the cursor or field level. As forward-only or fully scrollable. With the cursor keyset located on the server. Existing on either the server or the client. Concurrency In some multiuser applications it is very important for the data presented to the end user to be as current as possible. Position A cursor also keeps track of the current position in a result set. Scrollability The type of cursor employed by your application also affects the ability to move forward and backward through the rows in a result set; this is sometimes referred to as scrollability.
Is this page helpful? Yes No. Any additional feedback? Declare a cursor that defines a result set. Open the cursor to establish the result set. Fetch the data into local variables as needed from the cursor, one row at a time.
Close the cursor when done. I would argue you might want to use a cursor when you want to do comparisons of characteristics that are on different rows of the return set, or if you want to write a different output row format than a standard one in certain cases. Two examples come to mind:. One was in a college where each add and drop of a class had its own row in the table. It might have been bad design but you needed to compare across rows to know how many add and drop rows you had in order to determine whether the person was in the class or not.
I can't think of a straight forward way to do that with only sql. Another example is writing a journal total line for GL journals. You get an arbitrary number of debits and credits in your journal, you have many journals in your rowset return, and you want to write a journal total line every time you finish a journal to post it into a General Ledger.
By saying iterator I mean a way to traverse the record set aka a set of selected data rows and do operations on it while traversing. Hence you can use it for data retrieval for example.
Cursor works with the rows of the result set sequentially - row by row. A cursor can be viewed as a pointer to one row in a set of rows and can only reference one row at a time, but can move to other rows of the result set as needed. This link can has a clear explanation of its syntax and contains additional information plus examples.
Cursors can be used in Sprocs too. They are a shortcut that allow you to use one query to do a task instead of several queries. However, cursors recognize scope and are considered undefined out of the scope of the sproc and their operations execute within a single procedure.
In relational databases, operations are made on a set of rows. Sometimes the application logic needs to work with a row at a time rather than the entire result set at once. If you possess programming skills, you would probably use a loop like FOR or WHILE to iterate through one item at a time, do something with the data and the job is done.
But be advised, take this path and trouble may follow. Below, we will show some examples where using a CURSOR creates performance issues and we will see that the same job can be done in many other ways. After a short coffee break, the query finished executing, returning rows in the time shown below. In the cursor execution, we have two steps.
Step one, the positioning, when the cursor sets its position to a row from the result set. Step two, the retrieval, when it gets the data from that specific row in an operation called the FETCH. These operations are repeated until there are no more rows to work with. Our demo tables are relative small containing roughly 1, and rows.
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