Visão Geral

Curso Snowflake SQL, neste, Snowflake SQL, os alunos aprenderão Snowflake SQL, que é o SQL mais abrangente de qualquer banco de dados. Os alunos podem melhorar seu SQL enormemente por meio de centenas de exemplos usando os comandos Snowflake SQL. Nota: O que há de tão incrível no Snowflake SQL? Se você combinar os comandos SQL que funcionam em todos os fornecedores de banco de dados diferentes, ficará agradavelmente surpreso ao saber que quase todos os comandos SQL funcionam no Snowflake

Objetivo

Após realizar este Curso Snowflake SQL você será capaz de:

• Use o Nexus Chameleon para migrar e mover dados para o Snowflake.
• Use o Nexus para trabalhar com o banco de dados Snowflake para executar instruções SQL, recuperar análises avançadas e criar gráficos e tabelas.
• Aprenda uma ampla variedade de análises do Snowflake.
• Use métodos avançados de Ordenar por para classificar os conjuntos de respostas recuperados.
• Compreender e usar totalmente associações e subconsultas
• Interrogue os dados usando Case, Coalesce e Decode.
• Ser capaz de usar totalmente todas as diferentes opções temporárias do Snowflake.
• Crie tabelas e visualizações no sistema Snowflake.
• Utilize as diferentes funções de data do Snowflake.
• Use técnicas de agregação e agregação avançada.
• Manipule e manipule Strings.
• Aprenda e escreva SQL para funções agregadas estatísticas avançadas

Materiais

Conteúdo Programatico

Basic SQL Functions

1. Introduction
2. Setting Your Default Database and Schema
3. SELECT * (All Columns) in a Table
4. SELECT Specific Columns in a Table
5. Commas in the Front or Back?
6. Place your Commas in front for better Debugging Capabilities
7. Sort the Data with the ORDER BY Keyword
8. Use a Column Name or Number in an ORDER BY Statement
9. Two Examples of ORDER BY using Different Techniques
10. Changing the ORDER BY to Descending Order
11. NULL Values sort Last in Ascending Mode (Default)
12. Using the Nulls First Command
13. NULL Values sort First in Descending Mode (DESC)
14. Using the Nulls Last Command
15. Major Sort vs. Minor Sort
16. Multiple Sort Keys using Names vs. Numbers
17. Sorts are Alphabetical, NOT Logical
18. Using A Valued CASE Statement to Sort Logically
19. Using A Searched CASE Statement to Sort Logically
20. Quiz – Can you Add a Minor Sort?
21. Answer – Can you Add a Minor Sort?
22. Using Decode to Sort Logically
23. Quiz – Can you Add Two Minor Sorts Using Decode?
24. Answer – Can you Add Two Minor Sorts Using Decode?
25. How to ALIAS a Column Name
26. A Missing Comma can by Mistake become an Alias
27. Comments using Double Dashes are Single Line Comments
28. Comments for Multi-Lines
29. Comments for Multi-Lines as Double Dashes per Line
30. Comments are a Great Technique for Finding SQL Errors
31. Popular Snowflake Functions
32. Move Data to the Snowflake Effortlessly
33. Move Data to the Cloud Effortlessly

The WHERE Clause

1. The WHERE Clause limits Returning Rows
2. The WHERE Clause Needs Single-Quotes for Character Data
3. Using a Column ALIAS in the WHERE Clause
4. Numbers Don't Need Single or Double Quotes
5. Searching for NULL Values Using Equality Returns Nothing
6. Use IS NULL or IS NOT NULL when dealing with NULLs
7. Using Greater Than OR Equal To (>=)
8. AND in the WHERE Clause
9. Troubleshooting AND
10. OR in the WHERE Clause
11. Troubleshooting OR
12. WHY OR must utilize the Column Name Each Time
13. Troubleshooting Character Data
14. Using Different Columns in an AND Statement
15. Quiz – How many rows will return?
16. Answer to Quiz – How many rows will return?
17. What is the Order of Precedence?
18. Using Parentheses to change the Order of Precedence
19. Using an IN List in place of OR
20. The IN List is an Excellent Technique
21. IN List vs. OR brings the same Results
22. The IN List Can Use Character Data
23. Using a NOT IN List
24. Null Values in a NOT IN List Return No Rows
25. A Technique for Handling Nulls with a NOT IN List
26. The BETWEEN Statement is Inclusive
27. The NOT BETWEEN Statement is also Inclusive
28. The BETWEEN Statement Works for Character Data
29. LIKE uses Wildcards Percent ‘%’ and Underscore ‘_’
30. LIKE command Underscore is Wildcard for one Character
31. Using Upper and Lower to Handle Case Issues
32. Using ILIKE Handle Case Issues
33. Finding Anyone Who Name End in 'Y'
34. Escape Character in the LIKE Command changes Wildcards
35. Escape Characters Turn off Wildcards in the LIKE Command

Distinct, Group By, Top, and Pivot

1. The Distinct Command
2. Distinct vs. GROUP BY
3. Quiz – How many rows come back from the Distinct?
4. Answer – How many rows come back from the Distinct?
5. TOP Command
6. TOP Command is brilliant when ORDER BY is Used!
7. The FETCH Clause
8. Sample and Tablesample
9. TOP vs. Sample
10. The Pivot Command

Aggregation

1. Quiz – You calculate the Answer Set in your Mind
2. Answer – You calculate the Answer Set in your Mind
3. Quiz 2 – You calculate the Answer Set in your Mind
4. Answer Quiz 2 – You calculate the Answer Set in your Mind
5. There are Five Aggregates
6. Quiz – How many rows come back?
7. Answer – How many rows come back?
8. Troubleshooting Aggregates
9. GROUP BY when Aggregates and Normal Columns Mix
10. GROUP BY Delivers one row per Group
11. GROUP BY Dept_No or GROUP BY 1 the same thing
12. Limiting Rows and Improving Performance with WHERE
13. WHERE Clause in Aggregation limits unneeded Calculations
14. Keyword HAVING tests Aggregates after they are Totaled
15. Keyword HAVING is like an Extra WHERE Clause for Totals
16. Three types of Advanced Grouping
17. GROUP BY Grouping Sets
18. GROUP BY Grouping Sets Answer Set
19. GROUP BY Rollup
20. GROUP BY Rollup ResultSet
21. GROUP BY Cube
22. GROUP BY Cube ResultSet
23. Quiz - GROUP BY GROUPING SETS Challenge
24. Answer To Quiz - GROUP BY GROUPING SETS Challenge

Join Functions

1. A two-table join using Non-ANSI Syntax
2. A two-table join using Non-ANSI Syntax with Table Alias
3. You Can Fully Qualify All Columns
4. A two-table join using ANSI Syntax
5. Both Queries have the same Results and Performance
6. Quiz – Can You Finish the Join Syntax?
7. Answer to Quiz – Can You Finish the Join Syntax?
8. Quiz – Can You Find the Error?
9. Answer to Quiz – Can You Find the Error?
10. Super Quiz – Can You Find the Difficult Error?
11. Answer to Super Quiz – Can You Find the Difficult Error?
12. Quiz – Which rows from both tables Won’t Return?
13. Answer to Quiz – Which rows from both tables Won’t Return?
14. Left Outer Join
15. Left Outer Join Results
16. LEFT OUTER JOIN Using (+)
17. RIGHT OUTER JOIN
18. Right Outer Join Example and Results
19. RIGHT OUTER JOIN Using (+)
20. Full Outer Join
21. Full Outer Join Results
22. Which Tables are the Left, and which are the Right?
23. Answer - Which Tables are the Left and which are the Right?
24. INNER JOIN with Additional AND Clause
25. ANSI INNER JOIN with Additional AND Clause
26. ANSI INNER JOIN with Additional WHERE Clause
27. OUTER JOIN with Additional WHERE Clause
28. OUTER JOIN with Additional AND Clause
29. The DREADED Product Join
30. The DREADED Product Join Results
31. Cartesian Product Join with Traditional Syntax
32. Cartesian Product Join with ANSI Syntax
33. The CROSS JOIN
34. The CROSS JOIN Answer Set
35. The Self Join
36. An Associative Table is a Bridge that Joins Two Tables
37. Quiz – Can you Write the 3-Table Join with Traditional Join Syntax?
38. Answer to Quiz – Can you Write the 3-Table Join with Traditional Join Syntax?
39. Quiz – Can you Write the 3-Table Join Using ANSI Syntax?
40. Answer – Can you Write the 3-Table Join to ANSI Syntax?
41. Quiz – Can you Place the ON Clauses at the End?
42. Answer – Can you Place the ON Clauses at the End?
43. The 5-Table Join – Logical Insurance Model
44. Quiz - Write a Five Table Join Using ANSI Syntax
45. Answer - Write a Five Table Join Using ANSI Syntax
46. Quiz - Write a Five Table Join Using Traditional Syntax
47. Answer - Write a Five Table Join Using ANSI Syntax
48. Quiz –Re-Write this putting the ON clauses at the END
49. Answer –Re-Write this putting the ON clauses at the END

Date Functions

1. Current Date
2. Current_Date, Current_Time, and Current_Timestamp
3. Current_Time vs. LocalTime With Precision
4. Local_Time and Local_Timestamp With Precision
5. Add or Subtract Days from a date
6. The ADD_MONTHS Command
7. Using the ADD_MONTHS Command to Add 1 Year
8. Using the ADD_MONTHS Command to Add 5 Years
9. Formatting a Date Using the To_Char Command
10. Formatting Date and Time With To_Char
11. The To_Char command to format Dollar Signs
12. The To_Char Command for Formatting Numbers
13. The EXTRACT Command
14. EXTRACT from DATES and TIME
15. Using Extract
16. EXTRACT from DATES and TIME Optional Syntax
17. Another Option for Extracting Portions of Dates and Times
18. Using Date_Part to Extract
19. Implied Extract of Day, Month and Year using to_char
20. The Date_Part Function Using a Date
21. Great Date Functions to Know
22. DAYOFWEEK and a CASE Statement
23. Year and Days for the First/Last Weeks of the Year
24. First Day and Last Day Functions
25. Incrementing Date Values Using the Dateadd Function
26. Incrementing Time Values Using Dateadd
27. The Datediff command
28. The Datediff Function on Column Data
29. Calculating Days Between using the DATEDIFF Function
30. Changing the Date to a Timestamp
31. Find the First Day of the Current Month
32. Using Intervals
33. Using Day, Month, and Year Intervals
34. Complex Interval

Analytics

1. The Row_Number Command
2. Quiz – How did the Row_Number Reset?
3. Answer – How did the Row_Number Reset?
4. Find the Top Two Students Per Class_Code using Qualify
5. Find the Top Two Students using a Derived Table
6. The RANK Command
7. Getting RANK to Sort in DESC Order
8. RANK () OVER and PARTITION BY
9. RANK() OVER and a Qualify Statement
10. RANK() OVER and a WITH Derived Table
11. RANK vs. DENSE_RANK
12. DENSE_RANK() OVER and PARTITION BY
13. DENSE_RANK() OVER and QUALIFY
14. PERCENT_RANK () OVER with 14 rows in Calculation
15. PERCENT_RANK () OVER with 21 rows in Calculation
16. PERCENT_RANK() OVER and PARTITION BY
17. CSUM
18. CSUM – The Sort Explained
19. CSUM – Rows Unbounded Preceding Explained
20. The CSUM – Making Sense of the Data
21. CSUM – The Major and Minor Sort Key(s)
22. The ANSI OLAP – Reset with a PARTITION BY Statement
23. Totals and Subtotals through Partition BY
24. Moving SUM
25. Moving SUM every 3-rows Vs. a Continuous Average
26. Partition By Resets the Calculations
27. Moving Average
28. How the Moving Average Calculates
29. How the Sort works for Moving Average (MAVG)
30. Quiz – How is that Total Calculated?
31. Answer to Quiz – How is that Total Calculated?
32. Quiz – How is that 4th Row Calculated?
33. Answer to Quiz – How is that 4th Row Calculated?
34. Moving Average every 3-rows Vs. a Continuous Average
35. Partition BY Resets an ANSI OLAP
36. Moving Difference using ANSI Syntax
37. Moving Difference using ANSI Syntax with Partition By
38. Finding a Value of a Column in the Next Row with MIN
39. Finding a Value of a Column in the Next Row with PARTITION BY
40. Finding Multiple Values of a Column in Upcoming Rows
41. Finding The Next Date using MAX
42. COUNT OVER for a Sequential Number
43. COUNT OVER using ROWS UNBOUNDED PRECEDING
44. The MAX OVER Command
45. MAX OVER with PARTITION BY Reset
46. The MIN OVER Command
47. The MIN OVER Command with PARTITION BY
48. Finding Gaps Between Dates
49. The CSUM For Each Product_Id For The First 3 Days
50. Using FIRST_VALUE
51. FIRST_VALUE
52. FIRST_VALUE with Partitioning
53. FIRST_VALUE Combined with Row_Number and Qualify
54. FIRST_VALUE and Row_Number with a Derived Table
55. Using LEAD
56. Using LEAD with a PARTITION Statement
57. Using LEAD With an Offset of 2
58. Using LEAD With an Offset of 2 and a PARTITION
59. Using LAG
60. Using LAG with a PARTITION Statement
61. Using LAG With an Offset of 2
62. Using LAG With an Offset of 2 and a PARTITION
63. CUME_DIST
64. CUME_DIST With a Partition
65. CURRENT ROW AND UNBOUNDED FOLLOWING
66. Different Windowing Options
67. LISTAGG Basic Example
68. Another Example of LISTAGG
69. LISTAGG With a Pipe-Separated List
70. LISTAGG With a Comma-Separated List in Groups
71. MEDIAN Function
72. MEDIAN Example
73. MEDIAN with Partitioning and a WHERE Clause
74. MEDIAN with Partitioning
75. NTILE Function
76. How Ntile Works
77. Ntile
78. Ntile Continued
79. Ntile Percentile
80. Another Ntile Example
81. Using Quantiles (Partitions of Four)
82. NTILE Using a Value of 10
83. NTILE With a Partition
84. NTH_VALUE Function and Syntax
85. NTH_VALUE Arguments
86. NTH_VALUE
87. NTH_VALUE With Partition
88. NTH_VALUE With Partition and Ignore Nulls
89. PERCENTILE_CONT Function Description and Syntax
90. Final Result Information About PERCENTILE_CONT
91. PERCENTILE_CONT Function Arguments
92. PERCENTILE_CONT Example
93. PERCENTILE_CONT Example with Percentage Change
94. PERCENTILE_CONT With PARTITION Example
95. PERCENTILE_CONT With PARTITION and (0.4)
96. PERCENTILE_DISC Function Description and Syntax
97. PERCENTILE_DISC Function Arguments
98. PERCENTILE_DISC Example
99. PERCENTILE_DISC Example with Percentage Change
100. PERCENTILE_DISC With PARTITION Example
101. PERCENTILE_DISC With PARTITION and (0.4)
102. RATIO_TO_REPORT Function
103. RATIO_TO_REPORT Example
104. RATIO_TO_REPORT Example with Partitioning
105. SUM(SUM(n))

Temporary Tables

1. CREATING A Derived Table
2. Naming the Derived Table
3. Aliasing the Column Names in the Derived Table
4. CREATING A Derived Table using the WITH Command
5. Derived Query Examples with Three Different Techniques
6. Most Derived Tables Are Used To Join To Other Tables
7. The Three Components of a Derived Table
8. Visualize This Derived Table
9. Our Join Example With the WITH Syntax
10. An Example of Two Derived Tables in a Single Query
11. MULTIPLE Derived Tables using the WITH Command
12. WITH RECURSIVE Derived Table Hierarchy
13. WITH RECURSIVE Derived Table Query
14. WITH RECURSIVE Derived Table Definition
15. WITH RECURSIVE Derived Table Seeding
16. WITH RECURSIVE Derived Table Looping
17. WITH RECURSIVE Derived Table Looping in Slow Motion
18. WITH RECURSIVE Derived Table Looping Continued
19. WITH RECURSIVE Derived Table Ends the Looping
20. WITH RECURSIVE Derived Table Final Report
21. Creating a Temporary Table
22. Creating a Temporary Table using a CTAS
23. Dropping a Temporary Table

Sub-query Functions

1. An IN List is much like a Subquery
2. An IN List Never has Duplicates – Just like a Subquery
3. An IN List Ignores Duplicates
4. The Subquery
5. How a Basic Subquery Works
6. These are Equivalent Queries
7. The Final Answer Set from the Subquery
8. Quiz- Answer the Difficult Question
9. Answer to Quiz- Answer the Difficult Question
10. Should you use a Subquery of a Join?
11. Quiz - Write the Subquery
12. Answer to Quiz - Write the Subquery
13. Quiz - Write the More Difficult Subquery
14. Answer to Quiz - Write the More Difficult Subquery
15. Quiz – Write the Extreme Subquery
16. Answer To Quiz – Write the Extreme Subquery
17. Quiz - Write the Subquery with an Aggregate
18. Answer to Quiz - Write the Subquery with an Aggregate
19. Quiz - Write the Correlated Subquery
20. Answer to Quiz - Write the Correlated Subquery
21. The Basics of a Correlated Subquery
22. The Top Query always runs first in a Correlated Subquery
23. Correlated Subquery Example vs. a Join with a Derived Table
24. Quiz- A Second Chance to Write a Correlated Subquery
25. Answer - A Second Chance to Write a Correlated Subquery
26. Quiz- A Third Chance to Write a Correlated Subquery
27. Answer - A Third Chance to Write a Correlated Subquery
28. Quiz- Last Chance to Write a Correlated Subquery
29. Answer – Last Chance to Write a Correlated Subquery
30. Quiz – Write the Extreme Correlated Subquery
31. Answer – Write the Extreme Correlated Subquery
32. NOT IN Subquery Returns Nothing when NULLs are Present
33. Fixing a NOT IN Subquery with Null Values
34. Quiz- Write the NOT Subquery
35. Answer to Quiz- Write the NOT Subquery
36. Quiz - Write the Subquery using a WHERE Clause
37. Answer - Write the Subquery using a WHERE Clause
38. Quiz- Write the Subquery with Two Parameters
39. Answer to Quiz- Write the Subquery with Two Parameters
40. How the Double Parameter Subquery Works
41. More on how the Double Parameter Subquery Works
42. Quiz – Write the Triple Subquery
43. Answer to Quiz – Write the Triple Subquery
44. IN is equivalent to =ANY
45. Using a Correlated Exists
46. How a Correlated Exists matches up
47. The Correlated NOT Exists

Strings

1. UPPER and lower Functions
2. The Length Command Counts Characters
3. LENGTH Works on Fixed Length Columns
4. LENGTH and OCTET_LENGTH
5. The TRIM Command trims both Leading and Trailing Spaces
6. The RTRIM and LTRIM Command trims Spaces
7. Concatenation
8. Concatenation and SUBSTRING
9. Four Concatenations Together
10. LPAD and RPAD
11. The SUBSTR and SUBSTRING Command
12. How SUBSTR Works with NO ENDING POSITION
13. Using SUBSTR and LENGTH Together
14. The LEFT and RIGHT Functions
15. The POSITION Command finds a Letters Position
16. The POSITION Command is brilliant with SUBSTR
17. CHARINDEX Finds a Letter(s) Position in a String
18. The CHARINDEX Command is brilliant with SUBSTRING
19. The CHARINDEX Command Using a Literal
20. The REPLACE Function
21. REGEXP_REPLACE
22. REGEXP_INSTR
23. SOUNDEX Function to Find a Sound
24. The ASCII Function
25. The CHAR Function
26. The UNICODE Function
27. The Reverse String Function
28. The RIGHT Function

Interrogating the Data

1. Numeric Manipulation Functions
2. Finding the Cube Root
3. Ceiling Gets the Smallest Integer Not Smaller Than X
4. Floor Finds the Largest Integer Not Greater Than X
5. The Round Function and Precision
6. Quiz – Fill in the Answers for the NULLIF Command
7. Answer – Fill in the Answers for the NULLIF Command
8. The COALESCE Command
9. COALESCE is Equivalent to this CASE Statement
10. A Rounding Example Using CAST
11. CAST will Round Values up or Down
12. Valued Case vs. Searched Case
13. Combining Searched Case and Valued Case
14. Nested Case
15. The CASE Challenge
16. The CASE Challenge Answer
17. The Decode Command
18. A Trick for getting a Horizontal Case
19. Put a CASE in the ORDER BY
20. Using Decode to Sort Logically
21. Chapter 12 - View Functions
22. The Fundamentals of Views
23. Creating a Simple View to Restrict Sensitive Columns
24. Creating a Simple View to Restrict Rows
25. Creating a View to Join Tables Together
26. Join Views Allow Users to Merely Select Columns
27. Sometimes we Create Views for Formatting
28. Basic Rules for Views
29. How to Modify a View
30. The Exception to the ORDER BY Rule inside a View
31. Derived Columns in a View Should Contain a Column Alias
32. The Standard Way Most Aliasing is Done
33. Another Way to Alias Columns in a View CREATE
34. What Happens When a View Column gets Aliased Twice?
35. Aggregates on View Aggregates

UNION Set Operator

1. Rules of Set Operators
2. INTERSECT Explained Logically
3. INTERSECT Explained Logically
4. UNION Explained Logically
5. UNION Explained Logically
6. UNION ALL Explained Logically
7. UNION ALL Explained Logically
8. EXCEPT Explained Logically
9. EXCEPT Explained Logically
10. Minus Explained Logically
11. Minus Explained Logically
12. Testing Your Knowledge
13. Answer Testing Your Knowledge
14. Testing Your Knowledge
15. Answer Testing Your Knowledge
16. An Equal Number of Columns in both SELECT Lists
17. Columns in the SELECT list should be from the same Domain
18. The Top Query handles all Aliases
19. The Bottom Query does the ORDER BY
20. Great Trick: Place your Set Operator in a Derived Table
21. UNION vs. UNION ALL
22. Using UNION ALL and Literals
23. A Great Example of how EXCEPT works
24. USING Multiple SET Operators in a Single Request
25. Changing the Order of Precedence with Parentheses
26. Using UNION ALL for speed in Merging Data Sets
27. Using UNION to be same as GROUP BY GROUPING SETS

Creating Tables

1. Show Databases and Table DDL Commands
2. Finding Constraints
3. Create Table Syntax
4. Creating A Table in Snowflake
5. Creating Temporary and Transient Tables
6. Comparing Table Types
7. Data Types for Numeric, String, and Binary
8. Data Types for Date, Time, and Unstructured
9. Creating Tables with a Clustering Key
10. Joining Tables Can Have the Same Clustering Keys for Speed
11. Creating Tables with a Primary Key/Foreign Key Relationship
12. A Table with a NOT NULL Constraint
13. CREATE TABLE LIKE
14. CREATE a Temporary TABLE using LIKE
15. CREATE TABLE AS (CTAS) Populates the Table With Data
16. CREATE TABLE AS (CTAS) Can Choose Certain Columns
17. CREATE a Temporary Table AS (CTAS)
18. CREATE a Temporary Table AS (CTAS) Using a Join

Data Manipulation Language (DML)

1. INSERT Syntax # 1
2. INSERT Syntax # 2
3. INSERT Example with Multiple Rows
4. Inserting Null Values into a Table
5. INSERT/SELECT Command
6. INSERT/SELECT to Build a Data Mart
7. UPDATE Examples
8. Subquery UPDATE Command Syntax
9. Example of Subquery UPDATE Command
10. Deleting Rows in a Table

Statistical Aggregate Functions

1. The Stats Table
2. The KURTOSIS Function
3. A Kurtosis Example
4. The SKEW Function
5. A SKEW Example
6. The STDDEV_POP Function
7. A STDDEV_POP Example
8. The STDDEV_SAMP Function
9. A STDDEV_SAMP Example
10. The VAR_POP Function
11. A VAR_POP Example
12. The VAR_SAMP Function
13. A VAR_SAMP Example
14. The CORR Function
15. A CORR Example
16. Another CORR Example so you can Compare
17. The COVAR_POP Function
18. A COVAR_POP Example
19. Another COVAR_POP Example so you can Compare
20. The REGR_INTERCEPT Function
21. A REGR_INTERCEPT Example
22. Another REGR_INTERCEPT Example so you can Compare
23. The REGR_SLOPE Function
24. A REGR_SLOPE Example
25. Another REGR_SLOPE Example so you can Compare
26. The REGR_AVGX Function
27. A REGR_AVGX Example
28. Another REGR_AVGX Example so you can Compare
29. The REGR_AVGY Function
30. A REGR_AVGY Example
31. Another REGR_AVGY Example so you can Compare
32. The REGR_COUNT Function
33. A REGR_COUNT Example
34. The REGR_R2 Function
35. A REGR_R2 Example
36. The REGR_SXX Function
37. A REGR_SXX Example
38. The REGR_SXY Function
39. A REGR_SXY Example
40. The REGR_SYY Function
41. A REGR_SYY Example
42. Using GROUP BY