Filtering records with WHERE

Now let's transform the data by filtering out records with the WHERE clause

@searchlog = 
    EXTRACT UserId          int, 
            Start           DateTime, 
            Region          string, 
            Query           string, 
            Duration        int, 
            Urls            string, 
            ClickedUrls     string
    FROM "/SearchLog.tsv"
    USING Extractors.Tsv();

@output = 
    SELECT *
    FROM @searchlog
    WHERE Region == "en-gb";

OUTPUT @output 
    TO "/SearchLog_output.tsv"
    USING Outputters.Tsv();

The SQL Logical Operators

The AND/OR/NOT operators can be combined with parentheses to create more complex logical expressions

@output =
    SELECT Start, Region, Duration
    FROM @searchlog
    WHERE (Duration >= 2*60 AND Duration <= 5*60) OR NOT (Region == "en-gb");

The C# Logical Operators

U-SQL also supports the C# logical operators

@output =
 SELECT Start, Region, Duration
 FROM @searchlog
 WHERE (Duration >= 2*60 && Duration <= 5*60) || (!(Region == "en-gb"));

SQL Logical Operators (AND OR) versus C# Operators ( && || )

These operators behave the same except for their short-circuiting behavior:

  • SQL-style logical operators: These DO NOT short-circuit
  • C#-style logical operators: These DO short-circuit

Use the SQL-style logical operators unless you MUST have the short-circuiting behavior. The reasons why this is important are covered in a later chapter that describes the order of evaluation of predicates in expressions.

Find all the sessions occurring before a date

@output =
 SELECT Start, Region, Duration
 FROM @searchlog
 WHERE Start <= DateTime.Parse("2012/02/17");

Find all the sessions occurring between two dates

@output = 
  SELECT Start, Region, Duration
  FROM @searchlog
  WHERE 
   Start >= DateTime.Parse("2012/02/16") 
   AND Start <= DateTime.Parse("2012/02/17");

Filtering on calculated columns

Consider this case where SELECT is used to define a new column called DurationInMinutes

@output =
  SELECT 
      Start, 
      Region,
      Duration/60.0 AS DurationInMinutes
  FROM @searchlog;

There are a couple of approaches for filtering rows based on the DurationInMinutes value

The first option is to use RowSet refinement

@output =
  SELECT 
    Start,
    Region,
    Duration/60.0 ASDurationInMinutes
  FROM @searchlog;

@output =
  SELECT *
  FROM @output
  WHERE DurationInMinutes>= 20;

The second option is to repeat the expression in the WHERE clause

@output =
  SELECT 
    Start,
    Region,
    Duration/60.0 AS DurationInMinutes
  FROM @searchlog
    WHERE Duration/60.0>= 20;

WHERE does not work on calculated columns in the same statement

WHERE filters rows coming into to the statement. The DurationInMinutes column doesn't exist in the input. Therefore WHERE cannot operate on it. So, the example below will not compile

// SYNTAX ERROR: WHERE cannot be used with columns created by SELECT
@output =
  SELECT Start, Region,Duration/60.0 AS DurationInMinutes
  FROM @searchlog
  WHERE DurationInMinutes>= 20;

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