Where clause filtering in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the WHERE clause in a database query?

The WHERE clause filters rows in a table to return only those that meet a specified condition.

beginner

How does the WHERE clause affect the result of a SELECT query?

It limits the rows returned by the query to only those that satisfy the condition in the WHERE clause.

beginner

Which of these is a valid WHERE clause condition?
WHERE age > 30 or WHERE name = 'John'?

Both are valid. The first filters rows where age is greater than 30, the second filters rows where the name is exactly 'John'.

intermediate

Can the WHERE clause use multiple conditions? How?

Yes, by combining conditions with AND, OR, and parentheses to control logic, e.g., WHERE age > 20 AND city = 'Paris'.

beginner

What happens if you omit the WHERE clause in a SELECT query?

The query returns all rows from the table without filtering.

What does the WHERE clause do in a SQL query?

A Filters rows based on a condition

B Sorts the rows

C Joins two tables

D Deletes rows

The WHERE clause filters rows to include only those that meet the specified condition.

Which operator can be used to combine multiple conditions in a WHERE clause?

A AND

B JOIN

C GROUP BY

D ORDER BY

AND combines multiple conditions so all must be true for a row to be included.

What will this query return? SELECT * FROM users WHERE age < 18;

A Users older than 18

B Users younger than 18

C All users

D No users

The WHERE clause filters to include only users with age less than 18.

If you want to find rows where city is 'London' or 'Paris', which WHERE clause is correct?

A WHERE city = 'London' OR city = 'Paris'

B WHERE city = 'London' AND city = 'Paris'

C WHERE city IN ('London', 'Paris')

D Both A and D

Both A and D correctly filter rows where city is either London or Paris.

What happens if the WHERE clause condition is always false?

A Only the first row is returned

B All rows are returned

C No rows are returned

D An error occurs

If the condition is always false, no rows match and the result is empty.

Explain how the WHERE clause filters data in a database query.

Think about how you pick only certain items from a list based on a rule.

Describe how to combine multiple conditions in a WHERE clause and why you might do that.

Imagine choosing items that meet several rules at once or one of several rules.

Practice

1.

What does the WHERE clause do in a SQL query?

easy

A. Groups rows by a column

B. Filters rows based on a condition

C. Joins two tables together

D. Sorts the rows in ascending order

Solution

1. Step 1: Understand the purpose of WHERE clause

   The WHERE clause is used to select only rows that meet a specific condition.

2. Step 2: Compare with other SQL clauses

   Sorting is done by ORDER BY, joining by JOIN, grouping by GROUP BY, so WHERE is for filtering rows.

3. Final Answer:

   Filters rows based on a condition -> Option B

4. Quick Check:

   WHERE clause = filter rows [OK]

Hint: WHERE filters rows by condition, not sorting or joining [OK]

Common Mistakes:

• Confusing WHERE with ORDER BY
• Thinking WHERE joins tables
• Mixing WHERE with GROUP BY

2.

Which of the following is the correct syntax to filter rows where Age is greater than 30?

SELECT * FROM Users WHERE ___;

easy

A. Age > 30

B. Age = > 30

C. Age >> 30

D. Age >= 30

Solution

1. Step 1: Identify correct comparison operator

   The operator for 'greater than' is >, so 'Age > 30' is correct.

2. Step 2: Check other options for syntax errors

   'Age = > 30' and 'Age >> 30' are invalid syntax. 'Age >= 30' means 'greater or equal', not strictly greater.

3. Final Answer:

   Age > 30 -> Option A

4. Quick Check:

   Use > for greater than [OK]

Hint: Use > for greater than, >= for greater or equal [OK]

Common Mistakes:

• Using = > instead of >
• Using >> which is invalid
• Confusing > with >= operator

3.

Given the table Employees with columns Name and Salary, what rows will this query return?

SELECT Name FROM Employees WHERE Salary < 50000;

medium

A. Employees with salary less than 50000

B. Employees with salary greater than 50000

C. All employees regardless of salary

D. Employees with salary equal to 50000

Solution

1. Step 1: Understand the WHERE condition

   The condition Salary < 50000 means select rows where salary is less than 50000.

2. Step 2: Interpret the query result

   The query returns only the Name column for employees meeting that condition.

3. Final Answer:

   Employees with salary less than 50000 -> Option A

4. Quick Check:

   WHERE Salary < 50000 filters salaries below 50000 [OK]

Hint: Less than means <, so Salary < 50000 filters lower salaries [OK]

Common Mistakes:

• Confusing < with >
• Thinking it returns all employees
• Assuming it returns salary column too

4.

Identify the error in this query that tries to select users with age 18 or older:

SELECT * FROM Users WHERE Age => 18;

medium

A. WHERE clause cannot use numeric comparisons

B. Missing quotes around 18

C. The operator => is invalid; should be >= instead

D. SELECT * is not allowed with WHERE

Solution

1. Step 1: Check the comparison operator

   The operator => is not valid SQL syntax; the correct operator for 'greater or equal' is >=.

2. Step 2: Verify other parts of the query

   Numeric values like 18 do not need quotes, WHERE supports numeric comparisons, and SELECT * works with WHERE.

3. Final Answer:

   The operator => is invalid; should be >= instead -> Option C

4. Quick Check:

   Use >= for greater or equal, not => [OK]

Hint: Use >= for greater or equal, not => [OK]

Common Mistakes:

• Using => instead of >=
• Adding quotes around numbers
• Thinking WHERE can't compare numbers

5.

You want to select all products from a Products table where the Price is between 10 and 20 inclusive. Which WHERE clause is correct?

hard

A. Price BETWEEN 10 AND 20 EXCLUSIVE

B. Price > 10 AND Price < 20

C. Price >= 10 OR Price <= 20

D. Price >= 10 AND Price <= 20

Solution

1. Step 1: Understand inclusive range filtering

   Inclusive means including 10 and 20, so use >= and <= operators.

2. Step 2: Analyze each option

   Price > 10 AND Price < 20 excludes 10 and 20 (strictly greater and less). Price >= 10 OR Price <= 20 uses OR, which selects too many rows. Price BETWEEN 10 AND 20 EXCLUSIVE is invalid syntax.

3. Final Answer:

   Price >= 10 AND Price <= 20 -> Option D

4. Quick Check:

   Inclusive range uses >= and <= with AND [OK]

Hint: Use >= and <= with AND for inclusive ranges [OK]

Common Mistakes:

• Using > and < excludes boundary values
• Using OR instead of AND
• Trying invalid BETWEEN syntax