Operator precedence in WHERE in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select rows where age is greater than 30 and city is 'New York'.

SQL

SELECT * FROM users WHERE age [1] 30 AND city = 'New York';

Drag options to blanks, or click blank then click option'

A<

B<=

C>

D=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' instead of '>' which selects only age exactly 30.

Using '<' which selects ages less than 30.

The operator '>' means greater than, so age > 30 selects users older than 30.

2fill in blank

medium

Complete the code to select rows where status is 'active' or last_login is before 2023-01-01.

SQL

SELECT * FROM accounts WHERE status = 'active' OR last_login [1] '2023-01-01';

Drag options to blanks, or click blank then click option'

A>=

B!=

C=

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>=' which selects dates after or on 2023-01-01.

Using '=' which selects only logins exactly on 2023-01-01.

The operator '<' means before in dates, so last_login < '2023-01-01' selects older logins.

3fill in blank

hard

Fix the error in the WHERE clause to correctly select rows where score is greater than 80 and grade is 'A' or 'B'.

SQL

SELECT * FROM results WHERE score [1] 80 AND (grade = 'A' OR grade = 'B');

Drag options to blanks, or click blank then click option'

A>

B<

C=

D<=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' which selects scores less than 80.

Using '=' which selects scores exactly 80.

The operator '>' selects scores greater than 80. This is needed to filter high scores.

4fill in blank

hard

Fill both blanks to select rows where salary is greater than 50000 and department is 'Sales' or 'Marketing'.

SQL

SELECT * FROM employees WHERE salary [1] 50000 AND (department [2] 'Sales' OR department = 'Marketing');

Drag options to blanks, or click blank then click option'

A>

B<

C=

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' for salary which selects lower salaries.

Using '!=' for department which excludes 'Sales'.

Salary greater than 50000 uses '>'. Department equals 'Sales' uses '='.

5fill in blank

hard

Fill all three blanks to select rows where age is less than 40, city is 'Chicago', and status is not 'inactive'.

SQL

SELECT * FROM users WHERE age [1] 40 AND city [2] 'Chicago' AND status [3] 'inactive';

Drag options to blanks, or click blank then click option'

A>

B<

C=

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' for age which selects older people.

Using '=' for status which selects only 'inactive'.

Age less than 40 uses '<'. City equals 'Chicago' uses '='. Status not equal to 'inactive' uses '!='.

Practice

1. In SQL, which operator is evaluated first in a WHERE clause without parentheses: AND or OR?

easy

A. OR is evaluated before AND

B. AND is evaluated before OR

C. Both are evaluated at the same time

D. It depends on the database system

Solution

1. Step 1: Understand SQL operator precedence

   In SQL, AND has higher precedence than OR. This means AND conditions are evaluated before OR conditions when no parentheses are used.

2. Step 2: Apply to WHERE clause evaluation

   Without parentheses, SQL evaluates all AND conditions first, then applies OR. This affects which rows match the query.

3. Final Answer:

   AND is evaluated before OR -> Option B

4. Quick Check:

   Operator precedence: AND > OR [OK]

Hint: Remember: AND before OR unless parentheses change it [OK]

Common Mistakes:

• Thinking OR is evaluated before AND
• Assuming both have equal precedence
• Believing evaluation order depends on database

2. Which of the following WHERE clauses is syntactically correct in SQL?

easy

A. WHERE age > 30 OR AND salary < 50000 OR city = 'NY'

B. WHERE age > 30 AND OR salary < 50000 city = 'NY'

C. WHERE age > 30 salary < 50000 OR city = 'NY'

D. WHERE age > 30 AND salary < 50000 OR city = 'NY'

Solution

1. Step 1: Check logical operator usage

   WHERE age > 30 AND salary < 50000 OR city = 'NY' uses AND and OR correctly between conditions. Options A, B, and D have misplaced or missing operators.

2. Step 2: Validate syntax correctness

   SQL requires logical operators between conditions. WHERE age > 30 AND salary < 50000 OR city = 'NY' follows this rule, so it is syntactically valid.

3. Final Answer:

   WHERE age > 30 AND salary < 50000 OR city = 'NY' -> Option D

4. Quick Check:

   Logical operators must separate conditions [OK]

Hint: Operators must be between conditions, no missing or extra words [OK]

Common Mistakes:

• Placing AND or OR without conditions
• Missing operators between conditions
• Using operators in wrong order

3. Given the table Employees with columns age, salary, and city, what rows will this query return?

SELECT * FROM Employees WHERE age > 30 OR salary < 50000 AND city = 'NY';

medium

A. Rows where age > 30 or (salary < 50000 and city = 'NY')

B. Rows where (age > 30 or salary < 50000) and city = 'NY'

C. Rows where age > 30 and salary < 50000 and city = 'NY'

D. Rows where age > 30 and (salary < 50000 or city = 'NY')

Solution

1. Step 1: Apply operator precedence rules

   Since AND has higher precedence than OR, the condition is evaluated as: age > 30 OR (salary < 50000 AND city = 'NY').

2. Step 2: Interpret the WHERE clause

   The query returns rows where either age is greater than 30, or both salary is less than 50000 and city is 'NY'.

3. Final Answer:

   Rows where age > 30 or (salary < 50000 and city = 'NY') -> Option A

4. Quick Check:

   AND evaluated before OR [OK]

Hint: AND groups tighter than OR without parentheses [OK]

Common Mistakes:

• Ignoring operator precedence and grouping incorrectly
• Assuming OR groups before AND
• Misreading the condition order

4. Identify the error in this SQL query:

SELECT * FROM Employees WHERE age > 25 OR salary < 40000 AND;

medium

A. Missing FROM clause

B. Incorrect use of OR operator

C. Missing condition after AND operator

D. Incorrect column names

Solution

1. Step 1: Analyze the WHERE clause syntax

   The query ends with AND; which means the AND operator is not followed by a condition. This is a syntax error.

2. Step 2: Confirm other parts are correct

   The SELECT and FROM clauses are present and column names seem valid. The error is specifically the incomplete WHERE clause.

3. Final Answer:

   Missing condition after AND operator -> Option C

4. Quick Check:

   Logical operators must have conditions on both sides [OK]

Hint: Check that AND/OR always have conditions after them [OK]

Common Mistakes:

• Leaving dangling AND or OR without conditions
• Forgetting to complete WHERE clause
• Assuming syntax is correct despite missing parts

5. You want to select employees who are either older than 40 or work in 'LA' with a salary less than 60000. Which WHERE clause correctly uses operator precedence to get this result?

Options:
A) WHERE age > 40 AND salary < 60000 OR city = 'LA'
B) WHERE (age > 40 OR salary < 60000) AND city = 'LA'
C) WHERE age > 40 OR (salary < 60000 AND city = 'LA')
D) WHERE (age > 40 OR city = 'LA') AND salary < 60000

hard

A. WHERE age > 40 OR (salary < 60000 AND city = 'LA')

B. WHERE (age > 40 OR salary < 60000) AND city = 'LA'

C. WHERE age > 40 AND salary < 60000 OR city = 'LA'

D. WHERE (age > 40 OR city = 'LA') AND salary < 60000

Solution

1. Step 1: Understand the desired logic

   You want employees who are either older than 40, or who both work in 'LA' and have salary less than 60000.

2. Step 2: Apply operator precedence and parentheses

   Since AND has higher precedence, a clause like WHERE age > 40 AND salary < 60000 OR city = 'LA' groups as (age > 40 AND salary < 60000) OR city = 'LA'. But for our desired logic age > 40 OR (salary < 60000 AND city = 'LA'), we need explicit parentheses as in WHERE age > 40 OR (salary < 60000 AND city = 'LA').

3. Step 3: Compare options

   WHERE age > 40 AND salary < 60000 OR city = 'LA' groups as (age > 40 AND salary < 60000) OR city = 'LA' -- incorrect. WHERE (age > 40 OR salary < 60000) AND city = 'LA' requires city = 'LA' even for age > 40 -- incorrect. WHERE (age > 40 OR city = 'LA') AND salary < 60000 requires salary < 60000 always -- incorrect. WHERE age > 40 OR (salary < 60000 AND city = 'LA') correctly implements the logic.

4. Final Answer:

   WHERE age > 40 OR (salary < 60000 AND city = 'LA') -> Option A

5. Quick Check:

   Use parentheses to group AND conditions with OR correctly [OK]

Hint: Use parentheses to group AND conditions when mixed with OR [OK]

Common Mistakes:

• Not using parentheses and misreading condition grouping
• Assuming OR groups before AND
• Changing logic by wrong parentheses placement