Add new blog post: FLOAT Made My Dollars Float Away - FLOAT vs DECIMAL in MySQL (#91)

content/blogs/8. float-make-my-dollar-float-away.md

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+---
+title: FLOAT Made My Dollars Float Away - FLOAT vs DECIMAL in MySQL 💸
+date: 19th Sep 2025
+description: Recently I got a task Alter a table column from `FLOAT` to `DECIMAL(10,2)
+image: /blogs-img/blog8.png
+alt: FLOAT Made My Dollars Float Away - FLOAT vs DECIMAL in MySQL 💸
+ogImage: /blogs-img/blog8.png
+tags: ['mysql', 'float', 'decimal']
+published: true
+---
+
+### FLOAT Made My Dollars Float Away - FLOAT vs DECIMAL in MySQL
+
+Recently I got a task:
+
+> **"Alter a table column from `FLOAT` to `DECIMAL(10,2)`"**
+
+I thought:
+_"Pff, easy task. Just run an `ALTER TABLE` and done. Why is this even a ticket?"_
+
+But then I read the description.
+Turns out, **FLOAT was causing data loss**, and I needed to convert it without losing data.
+
+That’s when I realized: this isn’t just about one query. It’s about how **FLOAT silently eats your money** in MySQL.
+
+### Why FLOAT is a Problem
+
+`FLOAT` in MySQL is a **binary floating-point type**.
+It doesn’t store exact values — only approximations.
+
+That’s fine for rocket science 🚀 or graphics rendering 🎮, but for **money** where every cent matters? Disaster.
+
+Think of `FLOAT` as a leaky bucket. You pour in `$1,000,000.25`… and it gives you back `$999,999.94`.
+
+Not funny when it’s your salary.
+
+### Example of Data Loss
+
+```sql
+CREATE TABLE money_float (
+    id INT AUTO_INCREMENT PRIMARY KEY,
+    amount FLOAT
+);
+
+INSERT INTO money_float (amount) VALUES (1000000.25), (123456789.99);
+
+SELECT * FROM money_float;
+```
+
+Result:
+
+| id  | amount       |
+| --- | ------------ |
+| 1   | 1000000.25   |
+| 2   | 123456792.00 |
+
+We inserted `123456789.99`, but got back `123456792.00`.
+The bigger the number, the worse the corruption.
+
+### But Why Does FLOAT Lose Data?
+
+Here’s the fun part. Let’s make it simple.
+
+- `FLOAT` stores numbers in **binary (base 2)**.
+- But not every decimal number can be written exactly in binary.
+
+Example:
+
+- In decimal, `0.1` is simple.
+- In binary, `0.1` is **infinite repeating**: `0.0001100110011…`
+- So `FLOAT` cuts it off at some point and stores an approximation.
+
+That’s why when you do:
+
+```sql
+INSERT INTO money_float (amount) VALUES (0.1);
+SELECT amount FROM money_float;
+```
+
+You might see something like `0.10000000149`.
+
+Now imagine this tiny error repeated in **millions of dollars**.
+Errors pile up, and suddenly your 9-digit amount looks… off.
+
+### DECIMAL to the Rescue
+
+`DECIMAL` stores numbers differently:
+
+- Instead of binary approximation, it stores **exact digits as strings** internally.
+- That means `123456789.99` is stored as exactly `123456789.99`.
+
+```sql
+CREATE TABLE money_decimal (
+    id INT AUTO_INCREMENT PRIMARY KEY,
+    amount DECIMAL(15,2)
+);
+
+INSERT INTO money_decimal (amount) VALUES (1000000.25), (123456789.99);
+
+SELECT * FROM money_decimal;
+```
+
+Result:
+
+| id  | amount       |
+| --- | ------------ |
+| 1   | 1000000.25   |
+| 2   | 123456789.99 |
+
+Perfect. ✅ No rounding surprises.
+
+### Why ALTER Won’t Save You
+
+Here’s the trap I fell into:
+
+```sql
+ALTER TABLE money_float MODIFY amount DECIMAL(15,2);
+```
+
+You’d think this fixes it, right?
+Nope. ❌
+
+The data was already corrupted when it was first inserted as `FLOAT`.
+`ALTER` just moves the already-broken value into `DECIMAL`.
+
+Garbage in → garbage out.
+
+### Visual: FLOAT vs DECIMAL
+
+```
+FLOAT (approximation in binary):
+123456789.99  --->  123456792.00 💀
+
+DECIMAL (exact digits):
+123456789.99  --->  123456789.99 ✅
+```
+
+### Lessons Learned
+
+- Never use `FLOAT`/`DOUBLE` for money.
+- Always use `DECIMAL(precision, scale)` (e.g., `DECIMAL(15,2)`).
+- If your table already has money in `FLOAT`, you cannot fix the lost precision with `ALTER`. You’ll need to re-import or clean it at the source.
+
+### Final Thought
+
+Using `FLOAT` for money is like paying your salary in **Monopoly money**. 🎲💵
+It looks okay until you try to spend it — then you realize it’s worthless.
+
+Stick with `DECIMAL`, and your dollars will stay safe. ✅