[GitHub] [spark] beliefer opened a new pull request, #39667: [SPARK-42131][SQL] Extract the function that construct the select statement for JDBC dialect.

[GitBox]

beliefer opened a new pull request, #39667:
URL: https://github.com/apache/spark/pull/39667

   ### What changes were proposed in this pull request?
   Currently, JDBCRDD uses fixed format for SELECT statement.
   ```
   val sqlText = options.prepareQuery +
         s"SELECT $columnList FROM ${options.tableOrQuery} 
$myTableSampleClause" +
         s" $myWhereClause $getGroupByClause $getOrderByClause $myLimitClause 
$myOffsetClause"
   ```
   
   But some databases have different syntax that uses different keyword or 
sort. For example, MS SQL Server uses keyword TOP to describe LIMIT clause or 
Top N.
   The LIMIT clause of MS SQL Server.
   ```
   SELECT TOP(1) Model, Color, Price  
         FROM dbo.Cars  
         WHERE Color = 'blue'
   ```
   The Top N of MS SQL Server.
   ```
   SELECT TOP(1) Model, Color, Price  
   FROM dbo.Cars  
   WHERE Color = 'blue'  
   ORDER BY Price ASC
   ```
   
   Their PR will let JDBC dialect could define the special syntax own.
   
   ### Why are the changes needed?
   Extract the function that construct the select statement for JDBC dialect.
   
   
   ### Does this PR introduce _any_ user-facing change?
   'No'.
   New feature.
   
   
   ### How was this patch tested?
   N/A
   


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