SELECT tables continued....

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Questions and Answers

What is the purpose of the HAVING clause in SQL?

To filter the results after aggregation based on conditions (correct)

To filter individual rows before aggregation

To apply aggregate functions to each row

To combine multiple tables into one result set

Which of the following statements correctly uses GROUP BY in conjunction with HAVING?

SELECT course, MAX(max_enrl) FROM courses WHERE MAX(max_enrl) > 60

SELECT product_id, COUNT(*) FROM products GROUP BY product_id HAVING COUNT(*) > 1 (correct)

SELECT species, AVG(age) FROM animals HAVING AVG(age) > 10 GROUP BY species

SELECT product_id, SUM(unit_price) FROM products HAVING SUM(unit_price) > 10

Which aggregate function is used to find the total capacity of courses in a SQL query?

AVG()

SUM() (correct)

MAX()

COUNT()

What will the SELECT statement 'SELECT CONCAT(first_name, last_name) AS full_name FROM users;' return?

A single string combining first and last names Signup and view all the answers

Which of the following queries uses the HAVING clause correctly?

SELECT product_id, COUNT() FROM products GROUP BY product_id HAVING COUNT() < 5 Signup and view all the answers

In which part of a SQL query does the HAVING clause appear?

After the GROUP BY clause Signup and view all the answers

What does the COALESCE() function do in SQL?

Replaces NULL values with a specified value Signup and view all the answers

Which SQL operation is mandatory in a SELECT statement?

SELECT Signup and view all the answers

What is the primary purpose of the HAVING clause in SQL?

To filter records after aggregation Signup and view all the answers

Which operation allows you to group rows that have the same values in specified columns?

GROUP BY Signup and view all the answers

Which SQL function is used to return a single value summarizing multiple rows?

All of the above Signup and view all the answers

What does the LIKE operator do in SQL?

Filters data based on similarity to a pattern Signup and view all the answers

What is the result of using the BETWEEN clause in an SQL query?

Returns records whose values lie within a specified range Signup and view all the answers

How can you ensure that the results of an SQL query are unique?

Using the DISTINCT keyword Signup and view all the answers

Which clause is used to specify conditions for grouping in SQL?

HAVING Signup and view all the answers

What does the COUNT function do in SQL?

Counts the number of non-null values in a specified column Signup and view all the answers

Which wildcard character matches exactly one character in SQL string functions?

_ Signup and view all the answers

What is the primary function of the WHERE clause in SQL?

To apply conditions to filter records Signup and view all the answers

When using GROUP BY, what happens to the number of rows in the result set?

The number of rows decreases as duplicates are grouped. Signup and view all the answers

Which aggregate function would you use to calculate the average value in a specific column?

AVG Signup and view all the answers

What is the correct SQL statement to group data by 'category' and find the highest price in each category?

SELECT category, MAX(price) FROM table GROUP BY category; Signup and view all the answers

Why should the LIMIT clause always be used with an ORDER BY clause?

To ensure the order of returned rows is consistent. Signup and view all the answers

What does the GROUP BY clause do in a SQL query?

It groups rows sharing a property into summary rows. Signup and view all the answers

Which of the following is NOT an aggregate function in SQL?

FIND Signup and view all the answers

How does the ORDER BY clause behave by default when sorting rows?

In ascending order. Signup and view all the answers

What will happen if you omit the GROUP BY clause when using an aggregate function?

The aggregate function operates like it does with the WHERE clause. Signup and view all the answers

In a SQL query, what is the primary function of the LIMIT clause?

To specify the number of rows to return. Signup and view all the answers

Which of the following correctly describes a use case for the HAVING clause?

Filtering groups after aggregate functions are applied. Signup and view all the answers

What is the typical use of the GROUP BY clause in combination with aggregate functions?

To aggregate data and reduce row counts. Signup and view all the answers

Which SQL command is primarily used to summarize data across multiple rows?

GROUP BY Signup and view all the answers

How would you avoid getting unpredictable output when using LIMIT in a query?

Always combine it with an ORDER BY clause. Signup and view all the answers

Study Notes

Lecture 09: Select Statement and Clauses

The lecture covers Select Statements and Clauses in MySQL.
The outline includes Recap, Clauses, and More Functions.
The example provided shows a SELECT statement to retrieve specific columns (customer_id, first_name, last_name) from a customer table.

Select Statement Example

The example retrieves columns customer_id, first_name, and last_name.
The data is sourced from the "customer" table.
Sample data is provided, including customer IDs, names, and other data elements.

Select Statement - Select All

The example includes a query to select all columns from the "customer" table.
Data includes customer ID, first name, last name, joined date, birth date, phone, address, city, state, and customer points.
Data shows various values for each column illustrating the structure of the database table.

Clause - Order

The SELECT and FROM clauses are fundamental to a SELECT statement.
These two clauses are the only required components of a query.

Count and Distinct

The COUNT function returns the total number of rows in a table.
The DISTINCT keyword filters out duplicate values.

WHERE Clause - Condition

The WHERE clause filters results based on conditions.
It uses Boolean expressions and logical operators.
An example shows selecting rows where the phone number is not NULL to remove any missing data.

WHERE Clause - Operations

The WHERE clause allows various operations.
These operations include LIKE, BETWEEN, IN, NULL, comparisons, AND, and OR.

WHERE Clause - LIKE operation

The LIKE operation matches a string against a pattern.
Wildcard characters, %, are used to match any sequence of zero or more characters, and _, for any single character.

WHERE Clause - BETWEEN

The BETWEEN operator filters rows within a specified range.
It can be used with numbers and expressions.

WHERE Clause - IN

The IN operator filters rows based on a list.
It is used to compare values to a comma-separated list.

WHERE Clause - NULL

The IS NULL operator checks if a value is null ensuring correct handling of missing data.

WHERE Clause - Comparison Operators

Various comparison operators (e.g., =, <>, <, >, <=, >=) can be used with the WHERE clause to filter data.

WHERE Clause - AND

The AND operator combines multiple conditions in a WHERE clause, resulting in only rows matching all conditions.

WHERE Clause - OR

The OR operator allows multiple conditions to be present in WHERE clause, allowing a row to be included if any condition is met.

HAVING Clause

The HAVING clause filters groups of rows based on conditions.
It works in conjunction with the GROUP BY clause.

Functions - CONCAT()

The CONCAT() function concatenates strings into one.

Functions - GROUP_CONCAT()

The GROUP_CONCAT() function concatenates values from multiple rows within a group, defined by the GROUP BY clause.

Functions - SUBSTRING()

The SUBSTRING() function extracts a portion of a string.
Providing the starting position and the length.

Functions - COALESCE()

The COALESCE() function returns the first non-NULL value in a list of expressions, allowing to handle missing or null values (in tables).

Functions - ROUND()

The ROUND() function rounds a numeric value to a specific number of decimal places.

Order of Clauses

The standard order for SQL clauses:
SELECT
FROM
WHERE (optional)
GROUP BY (optional)
HAVING (optional)
ORDER BY (optional)
LIMIT (optional)

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