"SQL must know will be" | Lesson 21 study notes using a cursor

Lesson 21 using a cursor

This lesson will be taught what is 游标and how to use 游标.

21.1 Cursor

• SQLReturns a set of retrieval operation called 结果集rows, which rows are returned to the group of SQLstatement rows match (zero or more rows).

Sometimes, the need to move forward or backward in one or more rows retrieved row, which is where the use of the cursor.

• 游标cursorIs stored in a DBMSdatabase query on the server, it is not a SELECTstatement, but the statement is retrieved result set.
  After storing the cursor, the application may need to scroll or browse the data.

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Cursor options and features:

• It can be marked as read-only cursor, so that the data can be read, but not update, and delete.
• To control the orientation operation can be performed (forward, backward, first, last, absolute position, relative position, etc.).
• Some markers can be editable as, as some non-editable.
• A predetermined range, it causes the cursor to create a specific request (such as stored procedures) or accessible to all requests.
• Indicates the DBMSdata retrieved, (rather than point out a table activity data) to copy the data open and during the visit does not change the cursor.

Cursor is mainly used for interactive applications, where the user needs to scroll through the data on the screen, and the data can be browsed or make changes.

21.2 Use the cursor

Use the cursor involves several clear steps:

• Before using the cursor, you must declare (define) it. This process is not actually retrieve the data, it is only definition to use SELECTstatements and cursor options.
• Once declared, it must open a cursor for use. This process using the previously defined SELECTstatement to actual data retrieved.
• For cursors filled with data, taken as needed (retrieve) the rows.
• At the end of the cursor, you must close the cursor, if possible, release the cursor (depends on concrete DBMS).

After declaring a cursor, the cursor can be opened and closed frequently as needed. When the cursor is open, fetch operation may be performed as frequently as necessary.

21.2.1 create a cursor

• Use the DECLAREstatement to create a cursor.
• DECLARENamed cursor, and define the appropriate SELECTsentence, as required with WHEREother clauses.

21.2.2 Use the cursor

• Use the OPEN CURSORstatement to open a cursor.
• You can now FETCHaccess the data cursor statement. FETCHIndicate which rows to retrieve, from where to retrieve them and put them in where (such as variable names).

21.2.3 Close the cursor

SQL ServerAnd so DBMSit requires a clear release the resources occupied by the cursor.

• CLOSEStatement is used to close the cursor. Once the cursor is closed, if you do not open again, it will not be used. The second time using it do not need to declare, just OPENopen it can be.