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Summary - NULL handling

Write NULL-safe queries with COALESCE and NULLIF.

Summary - NULL handling

In this group you learned NULL, IS NULL, COALESCE, NULLIF and IFNULL. Handling NULL correctly is key to robust queries.

SELECT COALESCE(NULLIF(total, 0), 1) AS duzelti FROM orders;

If total is 0 returns NULL then 1.

COALESCE(NULLIF(total, 0), 1) AS duzelti is selected from orders.

Practice Summary - NULL handling

Select names from customers where email is null.

Starter code

query.sql
-- summary null
SELECT name
FROM customers
WHERE

Sample solution

query.sql
SELECT name
FROM customers
WHERE email IS NULL

Expected output

name
Mehmet
Zeynep

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 Which gives the first non-NULL value?

  • NULLIF
  • IFNULL
  • COALESCE Correct answer
  • IS NULL

Explanation: COALESCE returns the first non-NULL value.

2 Summary - NULL handling: match the terms

Term Meaning
IS NULL empty
COALESCE fallback
NULLIF null if equal
IFNULL two fallbacks

All lessons in Beginner

1 SELECT basics

  1. What is SELECT?
  2. Selecting all columns
  3. Selecting specific columns
  4. Column aliases with AS
  5. Literal values
  6. Summary - SELECT basics

2 WHERE filtering

  1. What is WHERE?
  2. Comparison operators
  3. AND and OR
  4. IN and BETWEEN
  5. Pattern matching with LIKE
  6. Summary - WHERE filtering

3 Ordering and limit

  1. ORDER BY
  2. ASC and DESC
  3. Multi-column ordering
  4. LIMIT
  5. OFFSET
  6. Summary - Ordering and limit

4 Project: CRUD app

  1. Project design
  2. Reading data
  3. Inserting data
  4. Updating data
  5. Deleting data

5 Data types

  1. Integer types
  2. Decimal types
  3. CHAR and VARCHAR
  4. DATE and TIME
  5. BOOLEAN
  6. Summary - Data types

6 Operators

  1. Arithmetic operators
  2. Comparison operators
  3. Logical operators
  4. IS NULL
  5. String concatenation
  6. Summary - Operators

7 String functions

  1. UPPER and LOWER
  2. LENGTH
  3. SUBSTRING
  4. TRIM
  5. CONCAT
  6. Summary - String functions

8 Project: Shopping cart

  1. Cart design
  2. Listing products
  3. Adding to cart
  4. Calculating total
  5. Removing from cart

9 Number functions

  1. ROUND
  2. CEIL and FLOOR
  3. ABS
  4. POWER and SQRT
  5. MOD
  6. Summary - Number functions

10 Date functions

  1. CURRENT_DATE and NOW
  2. EXTRACT
  3. Date arithmetic
  4. DATEDIFF
  5. Date formatting
  6. Summary - Date functions

11 NULL handling

  1. What is NULL?
  2. IS NULL
  3. COALESCE
  4. NULLIF
  5. Three-valued logic
  6. Summary - NULL handling

12 DISTINCT

  1. What is DISTINCT?
  2. DISTINCT on one column
  3. Multi-column DISTINCT
  4. DISTINCT with aggregates
  5. DISTINCT vs GROUP BY
  6. Summary - DISTINCT

13 INSERT

  1. INSERT basics
  2. Inserting into specific columns
  3. Multiple rows
  4. INSERT with SELECT
  5. DEFAULT values
  6. Summary - INSERT

14 UPDATE

  1. UPDATE basics
  2. Conditional UPDATE
  3. Multi-column UPDATE
  4. Calculated updates
  5. UPDATE pitfalls
  6. Summary - UPDATE

15 DELETE

  1. DELETE basics
  2. Conditional DELETE
  3. DELETE vs TRUNCATE
  4. Emptying a table
  5. Safe deletion
  6. Summary - DELETE

16 CREATE TABLE

  1. CREATE TABLE basics
  2. Column definitions
  3. NOT NULL
  4. DEFAULT
  5. PRIMARY KEY
  6. Summary - CREATE TABLE

17 Constraints

  1. UNIQUE constraint
  2. CHECK constraint
  3. FOREIGN KEY
  4. CASCADE
  5. Constraint management
  6. Summary - Constraints

18 Simple JOIN

  1. What is JOIN?
  2. INNER JOIN
  3. The ON clause
  4. Table aliases
  5. Multi-table JOIN
  6. Summary - Simple JOIN

19 Project: Inventory system

  1. Inventory design
  2. Product table
  3. Stock movements
  4. Stock report
  5. Low stock alert

20 Beginner final

  1. Customer query
  2. Order report
  3. Data update
  4. Table redesign