MySQL Filtering and Aggregating Data

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Questions and Answers

What is the purpose of the WHERE clause in SQL?

To aggregate data to calculate a total or average
To filter data based on a condition (correct)
To sort data in ascending or descending order
To group data based on a particular column

How can you get the total number of rows in a table?

Using the SELECT AVG(*) function
Using the SELECT GROUP(*) function
Using the SELECT COUNT(*) function (correct)
Using the SELECT SUM(*) function

What is the purpose of the SELECT SUM(col) function?

To get the maximum value of a column
To get the minimum value of a column
To get the total value of a column (correct)
To get the average value of a column

How do you get rows where a number is greater than or equal to a value?

Using the WHERE col >= n clause (A) Signup and view all the answers

What is the purpose of the SELECT AVG(col) function?

To get the average value of a column (B) Signup and view all the answers

How do you get rows where a number is greater than a value?

Using the WHERE col > n clause (B) Signup and view all the answers

What is the purpose of the SELECT COUNT(*) function?

To get the total number of rows in a table (C) Signup and view all the answers

How can you get the mean value of a column?

Using the SELECT AVG(col) function (C) Signup and view all the answers

What is the purpose of the ORDER BY clause in a SELECT statement?

To sort rows in a specific order (A) Signup and view all the answers

How do you get rows where values are missing in a specific column?

WHERE col IS NULL (B) Signup and view all the answers

What is the purpose of the LIMIT clause in a SELECT statement?

To limit the number of rows returned (D) Signup and view all the answers

How do you get unique values in a column?

SELECT DISTINCT col FROM table (A) Signup and view all the answers

What is the purpose of the WHERE clause in a SELECT statement?

To filter rows based on a condition (B) Signup and view all the answers

How do you get the current date and datetime?

SELECT CURDATE(), NOW(), CURTIME() (C) Signup and view all the answers

What is the purpose of the FROM clause in a SELECT statement?

To specify the table to retrieve data from (C) Signup and view all the answers

How do you get rows where values are not missing in a specific column?

WHERE col IS NOT NULL (C) Signup and view all the answers

What is the purpose of the BINARY operator in a WHERE clause?

To get case sensitive matching (D) Signup and view all the answers

What is the purpose of the SHOW TABLES command?

To list available tables (A) Signup and view all the answers

What is the purpose of the GROUP BY clause in SQL?

To group rows of a table based on one or more columns (D) Signup and view all the answers

What is the syntax to get rows where a number is between two values in SQL?

WHERE col BETWEEN m AND n (C) Signup and view all the answers

What is the purpose of the ORDER BY clause in SQL?

To sort data in ascending or descending order (A) Signup and view all the answers

How do you get rows where text contains specific letters in SQL?

WHERE col LIKE '%abc%' (C) Signup and view all the answers

What is the purpose of the HAVING clause in SQL?

To filter data based on specific conditions after grouping (D) Signup and view all the answers

How do you select all columns from a table in SQL?

SELECT * (B) Signup and view all the answers

What is the purpose of the WHERE clause in SQL?

To filter data based on specific conditions (C) Signup and view all the answers

How do you get rows where one condition and another condition holds in SQL?

WHERE condn1 AND condn2 (A) Signup and view all the answers

What is the purpose of the AS keyword in SQL?

To rename a column or alias (A) Signup and view all the answers

What is the syntax to get summaries grouped by values with a specific order in SQL?

GROUP BY col ORDER BY smmry DESC (A) Signup and view all the answers

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Study Notes

Filtering Data

Use WHERE clause to filter data based on conditions
Filter on numeric columns:
- WHERE col > n to get rows where a number is greater than a value
- WHERE col >= n to get rows where a number is greater than or equal to a value
- WHERE col BETWEEN m AND n to get rows where a number is between two values (inclusive)
Filter on text columns:
- WHERE col = 'x' to get rows where text is equal to a value
- WHERE col IN ('x', 'y') to get rows where text is one of several values
- WHERE col LIKE '%abc%' to get rows where text contains specific letters
Filter on multiple columns:
- WHERE condn1 AND condn2 to get rows where one condition and another condition holds

Aggregating Data

Use SELECT clause to aggregate data
Simple aggregations:
- SELECT COUNT(*) to get the total number of rows
- SELECT SUM(col) to get the total value of a column
- SELECT AVG(col) to get the mean value of a column
Get summaries grouped by values:
- SELECT col, COUNT(*) with GROUP BY col to get summaries grouped by values
- SELECT col, SUM(x) with GROUP BY col to get summaries grouped by values with sum
- SELECT col, COUNT(*) with GROUP BY col ORDER BY smmry DESC to get summaries grouped by values in order of summaries

Querying Tables

Get all columns from a table using SELECT *
Get a column from a table by name using SELECT col
Get multiple columns from a table by name using SELECT col1, col2
Override column names with SELECT col AS new_name
Arrange the rows in descending order of values in a column with ORDER BY col DESC
Limit the number of rows returned with LIMIT n

MySQL-Specific Syntax

Not all code works in every dialect of SQL
Use WHERE BINARY condn to get case sensitive matching
Get the current date with CURDATE() and the current datetime with NOW() or CURTIME()
List available tables with SHOW TABLES