Understanding Cursor Operations in SQL Server

As a technical blogger, I’d like to dive into the world of cursor operations in SQL Server. In this article, we’ll explore how to use cursors to fetch data from multiple tables and create insert statements for each table.

What are Cursors?

In SQL Server, a cursor is a control structure that allows you to iterate over a set of records (rows) within a database. It’s similar to a traditional loop in programming languages like C++ or Java. The main difference is that cursors operate on the database server-side, whereas loops execute on the client machine.

Types of Cursors

There are several types of cursors in SQL Server:

• Open Cursor: This cursor is opened and initialized before executing any commands.
• Fetch Cursor: This cursor fetches rows from a table or query that has been executed by an open cursor.
• Close Cursor: After fetching all rows, this cursor closes the connection to the database.

Using Cursors in SQL Server

Let’s examine how we can use cursors to achieve our goal of generating insert statements for multiple tables. The provided code snippet demonstrates a stored procedure called usp_InsertGenerator.

CREATE PROC [dbo].[usp_InsertGenerator]
AS
BEGIN
  DECLARE @tablename varchar(max)
  DECLARE @tbl table (insertVal varchar(max))
  DECLARE cursCol1 CURSOR FOR
  SELECT name FROM sys.tables 

  OPEN cursCol1
  FETCH NEXT FROM cursCol1 INTO @tablename
  WHILE @@FETCH_STATUS=0
  begin
      DECLARE cursCol CURSOR FOR
      SELECT column_name, data_type
      FROM information_schema.columns 
      WHERE table_name = @tableName

      OPEN cursCol
      DECLARE @string nvarchar(3000) 
      DECLARE @stringData nvarchar(3000)
      DECLARE @dataType nvarchar(1000)
      SET @string='INSERT '+@tableName+'('
      SET @stringData=''
      DECLARE @colName nvarchar(50)

      FETCH NEXT FROM cursCol INTO @colName,@dataType
      IF @@fetch_status<>0
      BEGIN
          PRINT 'Table '+@tableName+' not found, processing skipped.'
           FETCH NEXT FROM cursCol1 INTO @tablename
           CLOSE curscol
           DEALLOCATE curscol
           RETURN
      END
      WHILE @@FETCH_STATUS=0
      --FETCH NEXT FROM cursCol1 INTO @tablename
      BEGIN
          IF @dataType in ('varchar','char','nchar','nvarchar')
          BEGIN
              SET @stringData=@stringData+'''''''''+
                      isnull('+@colName+','''')+'''''',''+'
          END
          ELSE
          BEGIN
            SET @stringData=@stringData+''''''''"+
          isnull(cast('+@colName+' as varchar(200)),''0'')+'''''',''+'
          END

          SET @string=@string+@colName+','
          FETCH NEXT FROM cursCol INTO @colName ,@dataType
      END
        BEGIN
              DECLARE @Query nvarchar(4000)
              SET @query ='SELECT '''+substring(@string,0,len(@string)) + ') 
                  VALUES(''+ ' + substring(@stringData,0,len(@stringData)-2)+'''+'')'' 
                  FROM '+@tableName
              
              INSERT INTO @tbl (insertVal) EXEC (@query)
              -- PRINT (@query)
          END
      CLOSE cursCol
      DEALLOCATE cursCol
    FETCH NEXT FROM cursCol1 INTO @tablename
    END
    CLOSE cursCol1
    DEALLOCATE cursCol1  
    SELECT * FROM @tbl
END

Explanation of the Code

Here’s a step-by-step explanation of what the code does:

• We create two cursors: cursCol1 and cursCol.
• The outer cursor (cursCol1) fetches table names from the sys.tables system view.
• For each table, we open an inner cursor (cursCol) to fetch column information.
• We iterate through the columns of the current table using a loop. In this example, it’s assumed that there are only two types of data: varchar, char, nchar, and nvarchar. The code appends the column names to the @stringData variable, separated by commas.
• After fetching all column information for a table, we create an insert statement using the @query variable. We use the EXEC function to execute this query in the context of the current database connection (@tbl is used as a temporary table to store the results).
• Once we’ve generated the insert statements for all tables, we print their contents.

Advice and Best Practices

Here are some tips for using cursors effectively:

• Minimize cursor usage: Cursors can slow down your query performance. Avoid using them whenever possible.
• Use DECLARE @i INT = 1 instead of DECLARE cursCol CURSOR FOR: Declare the cursor variable before opening it.
• Close the cursor as soon as you’re done with it: Close the cursor after fetching all rows to prevent resource leaks.

By following these tips and understanding how to use cursors effectively, you can write more efficient and effective SQL Server code.

Last modified on 2024-07-10