Chapter 17 – Interrogating the Data

"The difference between genius and stupidity is that genius has its limits"

- Albert Einstein

Quiz – What would the Answer be?

Sample_Table

Class_Code

 

Grade_Pt

 Fr

 

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Can you guess what would return in the Answer Set?

Use the fake table above called Sample_Table, and try and predict what the Answer will be if this query was running on the system.

Answer to Quiz – What would the Answer be?

Sample_Table

Class_Code

 

Grade_Pt

 Fr

 

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Can you guess what would return in the Answer Set?

ErrorDivision by zero

You get an error when you DIVIDE by ZERO! Let’s turn the page and fix it!

The NULLIFZERO Command

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SELECT                  Class_Code

‚Grade_Pt / ( NULLIFZERO (Grade_pt) * 2 ) AS Math1

FROM  Sample_Table;

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What the NULLIFZERO does is make a zero into a NULL. So, the answer set you’d get from this is a simple ‘FR’, and then a NULL value represented usually by a ‘?’. If you have a calculation where a ZERO could kill the operation, and you don’t want that, you can use the NULLIFZERO command to convert any zero value to a NULL value.

Quiz – Fill in the Blank Values in the Answer Set

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SELECT   NULLIFZERO (Cust_No)        AS Cust_No

                ‚NULLIFZERO (Acc_Balance) AS Acc_Balance

                ‚NULLIFZERO (Location)        AS Location

FROM  Sample_Table ;

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Fill in the Answer Set above after looking at the table and the query.

Okay! Time to show me your brilliance! What would the Answer Set produce?

Answer to Quiz – Fill in the Blank Values in the Answer Set

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SELECT   NULLIFZERO (Cust_No)        AS Cust_No

                ‚NULLIFZERO (Acc_Balance) AS Acc_Balance

                ‚NULLIFZERO (Location)        AS Location

FROM  Sample_Table ;

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Here is the answer set! How’d you do? The NULLIFZERO command found a zero in Cust_No, so it made it Null. The others were not zero, so they retained their value. The only time NULLIFZERO changes data is if it finds a zero, and then it changes it to null.

Quiz – Fill in the Answers for the NULLIF Command

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SELECT   NULLIF(Cust_No, 0)         AS Cust1

                ‚NULLIF(Cust_No, 3)         AS Cust2

                ‚NULLIF(Acc_Balance, 0)  AS Acc1

                ‚NULLIF(Acc_Balance, 3)  AS Acc2

                ‚NULLIF(Location, 0)         AS Loc1

                ‚NULLIF(Location, 3)         AS Loc2

FROM Sample_Table;

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Fill in the Answer Set above after looking at the table and the query.

You can also use the NULLIF(). What you are asking Redshift to do is to NULL the answer if the COLUMN matches the number in the parentheses. What would the above Answer Set produce from your analysis?

Quiz – Fill in the Answers for the NULLIF Command

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SELECT   NULLIF(Cust_No, 0)         AS Cust1

                ‚NULLIF(Cust_No, 3)         AS Cust2

                ‚NULLIF(Acc_Balance, 0)  AS Acc1

                ‚NULLIF(Acc_Balance, 3)  AS Acc2

                ‚NULLIF(Location, 0)         AS Loc1

                ‚NULLIF(Location, 3)         AS Loc2

FROM Sample_Table;

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Look at the answers above, and if it doesn’t make sense, go over it again until it does.

The ZEROIFNULL Command

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SELECT   ZEROIFNULL (Cust_No) as Cust

                ‚ZEROIFNULL (Acc_Balance) as Balance

                ‚ZEROIFNULL (Location) as Location

FROM       Sample_Table;

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Fill in the Answer Set above after looking at the table and the query.

This is the ZEROIFNULL. What it will do is put a zero into a place where a NULL shows up. What would the Answer Set produce?

Answer to the ZEROIFNULL Question

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SELECT   ZEROIFNULL (Cust_No) as Cust

                 ,ZEROIFNULL (Acc_Balance) as Balance

                 ,ZEROIFNULL (Location) as Location

FROM     Sample_Table ;

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The answer set placed a zero in the place of the NULL Acc_Balance, but the other values didn’t change because they were NOT Null.

The COALESCE Command

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  SELECT Last_Name

  ,COALESCE (Home_Phone, Work_Phone, Cell_Phone) as Phone

  FROM Sample_Table ;

Last_Name  

Phone  

Fill in the Answer Set above after looking at the table and the query.

Coalesce returns the first non-Null value in a list, and if all values are Null, returns Null.

The COALESCE Answer Set

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SELECT Last_Name

 ,COALESCE (Home_Phone, Work_Phone, Cell_Phone) as Phone

FROM Sample_Table ;

Last_Name

 

Phone

 Jones

 

555-1234

 Patel

 

456-7890

 Gonzales

 

354-0987

 Nguyen

 

?

Coalesce returns the first non-Null value in a list, and if all values are Null, returns Null.

The Coalesce Quiz

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SELECT Last_Name

,COALESCE (Home_Phone, Work_Phone, Cell_Phone, 'No Phone') as Phone

FROM    Sample_Table ;

Last_Name

 

Phone

 

 

 

 

 

 

 

 

 

Fill in the Answer Set above after looking at the table and the query.

Coalesce returns the first non-Null value in a list, and if all values are Null, returns Null. Since we decided in the above query we don’t want NULLs, notice we have placed a literal ‘No Phone’ in the list. How will this effect the Answer Set?

Answer – The Coalesce Quiz

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SELECT Last_Name

  ‚COALESCE (Home_Phone, Work_Phone, Cell_Phone, 'No Phone') as Phone

FROM    Sample_Table ;

Last_Name

 

Phone

  Jones

 

555-1234

  Patel

 

456-7890

  Gonzales

 

354-0987

  Nguyen

 

No Phone

Answers are above! We put a literal in the list so there’s no chance of NULL returning.

The Basics of CAST (Convert And STore)

CAST will convert a column or value’s data type

temporarily into another data type. Below is the syntax:

SELECT CAST(<column-name> AS <data-type>[(<length>)] )

FROM  <table-name> ;

Examples using CAST:

CAST ( <smallint-data>  AS CHAR(5) )        /* convert smallint to character  */

CAST ( <decimal-data>  AS INTEGER )       /* truncates decimals    */

CAST ( <byteint-data>    AS SMALLINT )    /* convert binary to smallint    */

CAST ( <char-data>        AS BYTE (128) )    /* convert character to binary   */

CAST ( <byteint-data>   AS VARCHAR(5) )  /* convert byteint to character    */

CAST ( <integer-data>   AS FLOAT )             /* convert integer to float point   */

Data can be converted from one type to another by using the CAST function. As long as the data involved does not break any data rules (i.e. placing alphabetic or special characters into a numeric data type), the conversion works. The name of the CAST function comes from the Convert And STore operation that it performs.

Some Great CAST (Convert And STore) Examples

SELECT  CAST('ABCDE' AS CHAR(1) ) AS Trunc

,CAST(128 AS CHAR(3) ) AS OK

,CAST(127 AS INTEGER ) AS Bigger ;

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The first CAST truncates the five characters (left to right) to form the single character ‘A’. In the second CAST, the integer 128 is converted to three characters and left justified in the output. The 127 was initially stored in a SMALLINT (5 digits - up to 32767) and then converted to an INTEGER. Hence, it uses 11 character positions for its display, ten numeric digits and a sign (positive assumed) and right justified as numeric.

Some Great CAST (Convert And STore) Examples

SELECT   CAST(121.53 AS SMALLINT)  AS Whole

,CAST(121.53 AS DECIMAL(3,0)) AS Rounder ;

Whole  

 

  Rounder

121

 

122

The value of 121.53 was initially stored as a DECIMAL as 5 total digits with 2 of them to the right of the decimal point. Then, it is converted to a SMALLINT using CAST to remove the decimal positions. Therefore, it truncates data by stripping off the decimal portion. It does not round data using this data type. On the other hand, the CAST in the fifth column called Rounder is converted to a DECIMAL as 3 digits with no digits (3,0) to the right of the decimal, so it will round data values instead of truncating. Since .53 is greater than .5, it is rounded up to 122.

Some Great CAST (Convert And STore) Examples

SELECT Order_Number as OrdNo

,Customer_Number as CustNo

,Order_Date

,Order_Total

,CAST(Order_Total as integer)           as Chopped

,CAST(Order_Total as Decimal(5,0)) as Rounded

FROM Order_Table ;

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The Column Chopped takes Order_Total (a Decimal (10,2) and CASTs it as an integer which chops off the decimals. Rounded CASTs Order_Total as a Decimal (5,0), which takes the decimals and rounds up if the decimal is .50 or above.

The Basics of the CASE Statements

Sample_Table

Course_Name

 

Credits 

 

Tera-Tom on SQL

 

1

 

SELECT Course_Name

,CASE Credits

WHEN 1 THEN 'One Credit'

WHEN 2 THEN 'Two Credits'

WHEN 3 THEN 'Three Credits'

END AS CreditAlias

FROM Sample_Table ;

Course_Name

 

CreditAlias 

 

 

 

Fill in the Answer Set above after looking at the table and the query.

This is a CASE STATEMENT which allows you to evaluate a column in your table, and from that, come up with a new answer for your report. Every CASE begins with a CASE, and they all must end with a corresponding END. What would the answer be?

The Basics of the CASE Statement

Sample_Table

Course_Name

 

Credits 

 

Tera-Tom on SQL

 

1

 

SELECT Course_Name

,CASE Credits

WHEN 1 THEN 'One Credit'

WHEN 2 THEN 'Two Credits'

WHEN 3 THEN 'Three Credits'

END AS CreditAlias

FROM  Sample_Table ;

Course_Name     

 

CreditAlias

Tera-Tom on SQL

 

One Credit

This is a CASE STATEMENT which allows you to evaluate a column in your table, and from that, come up with a new answer for your report. Every CASE begins with a CASE, and they all must end with a corresponding END. What would the answer be?

Valued Case Vs. A Searched Case

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The second example is better unless you have a simple query like the first example.

Quiz - Valued Case Statement

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Look at the CASE Statement and look at the Course_Table, and fill in the Answer Set.

Answer - Valued Case Statement

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Above is the full answer set.

Quiz - Searched Case Statement

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Look at the CASE Statement and look at the Course_Table, and fill in the Answer Set.

Answer - Searched Case Statement

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Above is the full answer set.

Quiz - When NO ELSE is present in CASE Statement

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Notice now that we have a 4 under the ‘Credit’ Column. However, in our CASE statement, we don’t have instructions on what to do if the number is 4. What will occur?

Answer - When NO ELSE is present in CASE Statement

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A null value will occur when the evaluation falls through the case and there is no else statement. Notice above that we have a 4 under the ‘Credit’ Column. However, in our CASE statement, we don’t have instructions on what to do if the number is 4. That is why the null value is in the report.

When an ELSE is present in CASE Statement

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Notice now that we have a 4 under the ‘Credit’ Column. However, in our CASE statement, we don’t have instructions on what to do if the number is 4. What will occur?

Answer - When an ELSE is present in CASE Statement

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Since our value of 4 fell through the CASE statement, the ELSE statement kicked in and we delivered ‘Don’t Know’. Notice two single quotes that provided the word Don’t.

When an Alias is NOT used in a CASE Statement

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Notice now that we don’t have an ALIAS for the CASE Statement. What will the system place in there for the Column Title.

Answer - When an Alias is NOT used in a CASE Statement

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Notice now that we don’t have an ALIAS for the CASE Statement. The title given by default is < CASE Expression >. That is why you should ALIAS your Case statements.

Combining Searched Case and Valued Case

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This Query above uses both a Valued Case and Searched Case. That’s ALLOWED!

Nested Case

SELECT Last_Name

     ,CASE Class_Code

          WHEN 'JR' THEN 'Jr'

              ||(CASE WHEN Grade_pt < 2 THEN 'Failing'

                    WHEN Grade_pt < 3.5 THEN 'Passing'

                            ELSE 'Exceeding'

                END)

                     ELSE 'Sr'

                       ||(CASE WHEN Grade_pt < 2 THEN 'Failing'

                              WHEN Grade_pt < 3.5 THEN 'Passing'

                                    ELSE 'Exceeding'

                          END)

      END  AS  Status

FROM Student_Table WHERE Class_Code IN ('JR','SR')

ORDER BY Class_Code, Last_Name;

Last_Name

Status          

Bond

Jr Exceeding

McRoberts

Jr Failing

Delaney

Sr Passing

Phillips

Sr Passing

A NESTED Case occurs when you have a Case Statement within another CASE Statement. Notice the Double Pipe symbols (||) that provide Concatenation.

Put a CASE in the ORDER BY

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