SQL Assignment Help: Queries, Joins and Database Design

SQL assignment help is most useful when it explains why a query returns the rows it does. Most marks are lost to joins that drop or duplicate rows, and to aggregates grouped the wrong way.

This guide covers how SQL tasks are marked, the query patterns you will meet most, how SQL actually evaluates a query, and how to design and normalise a schema. Every example uses one small university database so you can follow along.

The Example Database

All the examples below use three tables. A student can enrol on many courses, and a course can have many students, so the enrolments table links them.

CREATE TABLE enrolments (
  student_id INT NOT NULL,
  course_id  INT NOT NULL,
  grade      NUMERIC(5, 2),
  PRIMARY KEY (student_id, course_id),
  FOREIGN KEY (student_id) REFERENCES students (student_id),
  FOREIGN KEY (course_id)  REFERENCES courses (course_id)
);

The composite primary key stops a student enrolling on the same course twice. The foreign keys stop enrolments that point at students or courses that do not exist.

How SQL Evaluates a Query

SQL is written in one order but processed in another. Knowing the logical order explains many confusing errors.

This is why WHERE cannot filter on an aggregate such as AVG(grade): grouping has not happened yet. Use HAVING instead. It is also why a column alias defined in SELECT works in ORDER BY but, in standard SQL, not in WHERE.

Joins: Inner, Left and the Rows You Lose

An inner join keeps only rows that match in both tables. A left join keeps every row from the left table and fills the right side with NULLs where there is no match.

SELECT c.course_id, c.title, COUNT(e.student_id) AS enrolled
FROM courses AS c
LEFT JOIN enrolments AS e ON e.course_id = c.course_id
GROUP BY c.course_id, c.title
ORDER BY enrolled DESC;

SELECT c.title
FROM courses AS c
LEFT JOIN enrolments AS e ON e.course_id = c.course_id
WHERE e.course_id IS NULL;

A common trap is adding a condition on the right-hand table in WHERE, such as WHERE e.grade > 60, after a LEFT JOIN. It removes the NULL rows and silently turns the left join back into an inner join. Put such conditions in the ON clause if you want to keep unmatched rows.

Aggregates with GROUP BY and HAVING

Aggregate functions such as COUNT, SUM, AVG, MIN and MAX collapse many rows into one per group. Every column in SELECT must either be inside an aggregate or listed in GROUP BY.

SELECT c.title, COUNT(e.grade) AS graded, ROUND(AVG(e.grade), 1) AS avg_grade
FROM courses AS c
JOIN enrolments AS e ON e.course_id = c.course_id
GROUP BY c.course_id, c.title
HAVING COUNT(e.grade) >= 5
ORDER BY avg_grade DESC;

Subqueries and Common Table Expressions

A subquery is a query inside another query. Common table expressions (CTEs), written with WITH, do the same job but are easier to read and debug.

WITH student_avg AS (
  SELECT student_id, AVG(grade) AS avg_grade
  FROM enrolments
  GROUP BY student_id
)
SELECT s.name, ROUND(sa.avg_grade, 1) AS avg_grade
FROM student_avg AS sa
JOIN students AS s ON s.student_id = sa.student_id
WHERE sa.avg_grade > (SELECT AVG(grade) FROM enrolments);

Be careful with NOT IN and subqueries. If the subquery returns even one NULL, NOT IN returns no rows at all. NOT EXISTS, or a LEFT JOIN with IS NULL, avoids the problem.

Window Functions

Window functions calculate across related rows without collapsing them into one. They answer questions such as "rank students within each course" that GROUP BY cannot.

SELECT course_id, student_id, grade,
       RANK() OVER (PARTITION BY course_id ORDER BY grade DESC) AS course_rank
FROM enrolments
WHERE grade IS NOT NULL;

RANK leaves gaps after ties (1, 1, 3), DENSE_RANK does not (1, 1, 2), and ROW_NUMBER gives every row a unique number. Pick the one the question implies.

NULLs and Three-Valued Logic

NULL means unknown or missing, not zero or an empty string. Any comparison with NULL, including NULL = NULL, gives unknown rather than true, so WHERE drops the row.

• Use IS NULL and IS NOT NULL, never = NULL.
• COUNT(*) counts rows; COUNT(column) skips NULLs.
• Aggregates such as AVG and SUM ignore NULLs.
• COALESCE(grade, 0) replaces NULL with a value, but only do this if zero is genuinely correct.

Database Design and Normalisation

Design tasks ask you to turn a description into tables, keys and relationships, usually with an entity-relationship diagram. Normalisation removes redundancy so that each fact is stored once.

Many-to-many relationships always need a linking table, like enrolments here. In an ERD, show each relationship's cardinality and say which side holds the foreign key.

Dialects and Safe Queries

Core SQL is standard, but databases differ in the details. Say which system your solution targets.

• Limiting rows: LIMIT in MySQL, PostgreSQL and SQLite; TOP in SQL Server; FETCH FIRST n ROWS ONLY in standard SQL, Oracle and PostgreSQL.
• Strings: use single quotes for text values, as in WHERE name = 'Ada'.
• Application code: never build queries by joining user input into a string. Use parameters to prevent SQL injection.

# Python with sqlite3: the ? placeholder is filled safely
cur.execute("SELECT name FROM students WHERE student_id = ?", (student_id,))

Common SQL Mistakes

How STEM Donkey Helps with SQL Assignments

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