KwickCards SQL and databases

SQL and databases: 18 revision cards

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SQL for CBSE

MySQL for Class 11-12

SQL is easy marks when you practise clause by clause.

SQL questions are some of the easiest marks in CBSE CS and IP, if you practise the right way.

SELECT, WHERE, GROUP BY, HAVING, aggregate functions, joins, keys and NULL behaviour: Kajal Ma'am teaches each clause with real tables and exam-style queries, so you know exactly what a query returns before you write the answer.

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CBSE CS · IP · SQL

GROUP BY then HAVING

6 rows in emp
GROUP BY dept
HAVING COUNT(*) > 1
ORDER BY dept

GROUP BY and HAVING together, a favourite in CBSE board papers.

Result:

HR | 2

IT | 3

Rows are grouped by dept: IT has 3 rows, HR has 2, Sales has 1. HAVING COUNT(*) > 1 removes the Sales group. ORDER BY dept sorts the result alphabetically.

Remember: WHERE filters rows before grouping, HAVING filters groups after.

CBSE CS · IP · SQL

How NULL changes aggregates

row	bonus
1	500
2	NULL
3	300
4	NULL

Predict COUNT(*), COUNT(bonus), SUM, AVG

COUNT(*) and COUNT(bonus) on the same table give different answers. Here is why.

Answers: 4, 2, 800, 400

COUNT(*) counts all rows: 4. COUNT(bonus) counts only non-NULL values: 2. SUM(bonus) adds 500 + 300 = 800. AVG(bonus) divides by the non-NULL count, so 800 / 2 = 400 (MySQL may display it as 400.0000).

Aggregate functions other than COUNT(*) ignore NULL values.

CBSE CS · IP · SQL JOINS

An equi-join between two tables

How many rows are returned?

-- student(roll,name): (1,Asha),(2,Ravi)
-- result(roll,marks): (1,88),(3,75)
SELECT s.name, r.marks
FROM student s JOIN result r
ON s.roll = r.roll;

Joins combine rows from two tables using a common column.

Answer: 1 row, Asha | 88.

Only roll 1 appears in both tables, so only Asha is matched. Ravi (roll 2) has no result row, and roll 3 in result has no student, so neither appears.

This is an equi-join: rows are matched with the = condition on the common column roll. Use table aliases like s and r to keep queries short and clear.

SQL NULL

Why = NULL never works

Do

WHERE phone IS NULL
IS NOT NULL for values

Don't

WHERE phone = NULL
Treat NULL like a value

If your query uses = NULL, it will not find the rows you expect.

NULL means an unknown value, so WHERE phone = NULL is never true and SELECT COUNT(*) with it returns 0, even when some phones are missing. The IS NULL operator is the correct way to test for missing values: WHERE phone IS NULL finds them.

Use IS NOT NULL to find rows that do have a value.

SQL COMMANDS

Which of these is a DDL command?

INSERT
UPDATE
ALTER
DELETE

DDL or DML? Sort these commands in five seconds.

Answer: C) ALTER.

DDL (Data Definition Language) changes the structure of the database: CREATE, ALTER, DROP. DML (Data Manipulation Language) works on the data inside tables: INSERT, UPDATE, DELETE and SELECT.

A CBSE favourite: DELETE removes rows, DROP removes the whole table. Learn three examples of each type and this question becomes a sure mark.

SQL · AGGREGATES

NULL inside AVG()

What does it return?

-- marks: 10, NULL, 20
SELECT AVG(marks)
FROM student;

How does AVG() treat a NULL value?

Answer: 15.

Aggregate functions ignore NULL. AVG adds the non-NULL values, 10 + 20 = 30, and divides by the count of non-NULL values, 2, giving 15 (MySQL may show 15.0000).

If NULL were treated as 0, the answer would be 10, a very common mistake. Always check a column for NULL values before calculating any aggregate by hand.

SQL · GROUP BY

Which clause filters groups after GROUP BY?

WHERE
HAVING
ORDER BY
DISTINCT

WHERE or HAVING, which one belongs after GROUP BY?

Answer: B) HAVING.

WHERE filters individual rows before they are grouped, so it cannot use aggregate functions. HAVING filters the groups after GROUP BY and can use conditions like COUNT(*) > 2 or AVG(marks) >= 60.

Remember the full clause order: SELECT, FROM, WHERE, GROUP BY, HAVING, ORDER BY.

SQL KEYS

Unique and NOT NULL, every row.

Primary key with NULL? Never.

Keys are basic, but this question still trips students up.

Can a primary key hold NULL? No, never.

A primary key uniquely identifies each row, so its values must be unique and NOT NULL. A table can have only one primary key, though it may be made of more than one column.

A candidate key that is not chosen as the primary key is called an alternate key.

SQL Revision

SQL aggregate functions

Aggregates

COUNT()
SUM()
AVG()
MAX()
MIN()

Aggregate functions return one value from many rows.

These five functions appear in almost every CBSE CS and IP SQL question, often with GROUP BY.

Key rule: all of them ignore NULL values, except COUNT(*), which counts every row.

MAX() and MIN() also work on text and dates, not only numbers.

Practise writing queries like: SELECT class, AVG(marks) FROM student GROUP BY class;

SQL · KEYS

Candidate, primary, alternate keys

Candidate keys
Primary key
Alternate keys

Keys are a very common theory question in CBSE Computer Science.

Think of a student table. Both admission number and Aadhaar number could identify a student uniquely, both are candidate keys. If you choose admission number as the primary key, Aadhaar number becomes an alternate key.

In a separate fees table, admission number would act as a foreign key that links back to the student table.

SQL · SELECT

SQL clauses in order

SELECT columns
FROM table
WHERE rows
GROUP BY column
HAVING groups
ORDER BY

Writing clauses in the wrong order gives a syntax error.

Remember this sequence for every SELECT query: SELECT, FROM, WHERE, GROUP BY, HAVING, ORDER BY.

Not every query needs all the clauses, but the ones you use must follow this order.

Example: SELECT dept, AVG(salary) FROM emp WHERE salary > 20000 GROUP BY dept HAVING COUNT(*) > 2 ORDER BY dept;

SQL · NULL

NULL: Unknown, not 0 or blank. Test it with IS NULL. Arithmetic with NULL gives NULL.

ExampleWHERE marks IS NULL

NULL behaviour is a small topic with many exam questions.

Because NULL is unknown, a condition like marks = NULL is never true, use IS NULL instead. An expression like salary + NULL also results in NULL.

Aggregate functions skip NULL values, which changes averages and counts.

Master these rules and you will handle NULL-based output questions with ease.

SQL COMMANDS

DDL

Defines table structure
CREATE, ALTER, DROP

DML

Works on rows
INSERT, UPDATE, DELETE

DDL or DML? This difference is asked in almost every CBSE paper.

DDL (Data Definition Language) commands shape the database, creating a table, adding a column, or dropping a table. DML (Data Manipulation Language) commands handle the records inside those tables.

A quick test: if the command changes what the table looks like, it is DDL. If it changes or retrieves the rows, it is DML.

SQL · FILTERING

WHERE first, HAVING later

All rows
WHERE
GROUP BY
HAVING

Both filter data, but at different stages of the query.

WHERE decides which rows enter the grouping. HAVING decides which groups appear in the final result.

You can use both in one query: SELECT class, AVG(marks) FROM student WHERE gender = 'F' GROUP BY class HAVING AVG(marks) > 70;

That query first keeps only female students, groups them by class, and then shows classes with an average above 70.

SQL Myth

Is NULL the same as zero?

NULL means unknown or missing
SUM, AVG, COUNT(col) skip NULL

Myth: NULL is the same as 0 or an empty string.

One of the most common SQL misconceptions among students.

If a student's marks are NULL, it does not mean they scored zero, it means the marks are not recorded. That is why AVG(marks) ignores NULL rows and why marks = 0 will not find them.

Use IS NULL to find missing values, and remember that any arithmetic with NULL gives NULL.

SQL · COUNT

Why COUNT(phone) is 7, not 10

Phone recorded
Phone NULL

Same function, different results. Here is why.

Suppose a table has 10 students and 3 of them have no phone number recorded. COUNT(*) returns 10, but COUNT(phone) returns 7.

CBSE output questions love this difference, especially when a table shows NULL in one column.

Read the table carefully before you write the count, look for NULL values first.

SQL · QUERIES

Any SQL query in 5 steps

Read the table, spot NULLs
Pick SELECT columns
Add WHERE
GROUP BY + HAVING
ORDER BY, end with ;

A simple method for writing SQL queries without panic.

CBSE board questions give a table and ask you to write queries or predict their output. Start by understanding the table itself, column names, data types and any NULL values.

Then build the query clause by clause in the correct order.

For output questions, work through the same steps in reverse: filter rows, form groups, apply HAVING, then sort.

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Written by Kajal Mehta (Kajal Ma'am), MCA, teaching computer subjects since 2004. All KwickCards