Decimal and floating-point types in MySQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main difference between DECIMAL and FLOAT types in MySQL?

DECIMAL stores exact numeric values with fixed precision and scale, ideal for financial data. FLOAT stores approximate values using floating-point representation, which can introduce rounding errors.

beginner

How do you define a DECIMAL column with 10 digits total and 2 digits after the decimal point?

Use DECIMAL(10,2). This means 10 digits total, with 2 digits to the right of the decimal point.

intermediate

Why might FLOAT or DOUBLE types cause rounding errors in calculations?

FLOAT and DOUBLE use binary floating-point representation, which cannot exactly represent some decimal fractions, causing small rounding errors in calculations.

beginner

Which data type should you use for storing currency values in MySQL and why?

Use DECIMAL because it stores exact values without rounding errors, which is important for accurate financial calculations.

intermediate

What happens if you insert a value with more decimal places than defined in a DECIMAL column?

MySQL rounds the value to fit the defined scale (number of decimal places). For example, inserting 12.345 into DECIMAL(5,2) stores 12.35.

Which MySQL data type stores exact numeric values with fixed precision?

A VARCHAR

B FLOAT

C DECIMAL

D DOUBLE

DECIMAL stores exact numeric values with fixed precision and scale.

What does DECIMAL(8,3) mean in MySQL?

A 8 digits total, 3 digits after decimal point

B 3 digits total, 8 digits after decimal point

C 8 digits after decimal point

D 3 digits total

DECIMAL(8,3) means 8 digits total with 3 digits after the decimal point.

Which data type is more likely to cause rounding errors?

A DATE

B DECIMAL

C CHAR

D FLOAT

FLOAT uses approximate floating-point representation, which can cause rounding errors.

If you need to store money values accurately, which type should you choose?

A DECIMAL

B DOUBLE

C FLOAT

D TEXT

DECIMAL stores exact values, making it best for money.

What happens if you insert 123.4567 into DECIMAL(6,2)?

A Stored as 123.45

B Stored as 123.46

C Error: too many decimal places

D Stored as 123.4567

MySQL rounds to fit the scale, so 123.4567 becomes 123.46.

Explain the difference between DECIMAL and FLOAT types in MySQL and when to use each.

Think about precision and rounding.

Describe how MySQL handles values with more decimal places than defined in a DECIMAL column.

Consider what happens to extra digits.

Practice

1. Which MySQL data type should you use to store exact monetary values without rounding errors?

easy

A. VARCHAR

B. FLOAT

C. DECIMAL

D. DOUBLE

Solution

1. Step 1: Understand the purpose of DECIMAL

   DECIMAL stores exact decimal numbers with fixed digits, ideal for money.

2. Step 2: Compare with FLOAT and DOUBLE

   FLOAT and DOUBLE store approximate values, which can cause rounding errors.

3. Final Answer:

   DECIMAL -> Option C

4. Quick Check:

   Exact decimal storage = DECIMAL [OK]

Hint: Use DECIMAL for exact money values, not FLOAT or DOUBLE [OK]

Common Mistakes:

• Choosing FLOAT or DOUBLE for money causing rounding errors
• Using VARCHAR which stores text, not numbers

2. Which of the following is the correct syntax to define a DECIMAL column with 8 digits total and 2 digits after the decimal point in MySQL?

easy

A. DECIMAL(8,2)

B. DECIMAL(2,8)

C. DECIMAL(10,2)

D. DECIMAL(8)

Solution

1. Step 1: Recall DECIMAL syntax

   DECIMAL(M,D) means M total digits, D digits after decimal point.

2. Step 2: Match given digits

   We want 8 total digits and 2 after decimal, so DECIMAL(8,2) is correct.

3. Final Answer:

   DECIMAL(8,2) -> Option A

4. Quick Check:

   DECIMAL(total, decimals) = DECIMAL(8,2) [OK]

Hint: DECIMAL(M,D): M=total digits, D=digits after decimal [OK]

Common Mistakes:

• Swapping M and D values
• Using DECIMAL(8) which lacks decimal precision

3. Given the table prices with a column cost FLOAT(7,2), what will be the result of inserting 12345.6789 and then selecting cost?

medium

A. 12345.679

B. 12345.68

C. 12345.67

D. 12345.000

Solution

1. Step 1: Understand FLOAT(7,2) meaning

   FLOAT(7,2) means 7 digits total, 2 after decimal, but FLOAT is approximate.

2. Step 2: Insert and round value

   12345.6789 rounds to 12345.68 when stored with 2 decimals.

3. Final Answer:

   12345.68 -> Option B

4. Quick Check:

   FLOAT rounds to 2 decimals = 12345.68 [OK]

Hint: FLOAT rounds approximate values to specified decimals [OK]

Common Mistakes:

• Expecting exact storage like DECIMAL
• Ignoring rounding behavior of FLOAT

4. You defined a column as FLOAT(5,2) but notice values like 1234.567 are stored as 1234.56. What is the likely cause and how to fix it?

medium

A. MySQL truncates decimals by default; enable rounding mode

B. FLOAT(5,2) syntax is invalid; use FLOAT without parameters

C. The value exceeds FLOAT range; use DOUBLE instead

D. FLOAT stores approximate values; use DECIMAL(7,3) for exact decimals

Solution

1. Step 1: Recognize FLOAT behavior

   FLOAT stores approximate values and rounds to specified decimals, causing truncation.

2. Step 2: Choose correct type for exact decimals

   DECIMAL stores exact decimals and preserves precision, so use DECIMAL(7,3).

3. Final Answer:

   FLOAT stores approximate values; use DECIMAL(7,3) for exact decimals -> Option D

4. Quick Check:

   Approximate FLOAT -> switch to DECIMAL for precision [OK]

Hint: Use DECIMAL for exact decimals, FLOAT is approximate [OK]

Common Mistakes:

• Thinking FLOAT(5,2) is invalid syntax
• Assuming DOUBLE fixes decimal truncation
• Expecting MySQL to round automatically without changing type

5. You need to store scientific measurements with very large or very small values and some decimal precision. Which MySQL data type is best and why?

hard

A. FLOAT or DOUBLE, because they store approximate floating-point numbers with wide range

B. DECIMAL, because it stores exact fixed-point numbers

C. VARCHAR, to store numbers as text for precision

D. INT, because it stores whole numbers efficiently

Solution

1. Step 1: Understand scientific measurement needs

   Scientific data often requires wide range and floating precision, not exact fixed decimals.

2. Step 2: Choose FLOAT or DOUBLE for wide range

   FLOAT and DOUBLE support very large/small values with approximate precision, ideal for science.

3. Final Answer:

   FLOAT or DOUBLE, because they store approximate floating-point numbers with wide range -> Option A

4. Quick Check:

   Wide range + approximate decimals = FLOAT/DOUBLE [OK]

Hint: Use FLOAT/DOUBLE for scientific data, DECIMAL for money [OK]

Common Mistakes:

• Choosing DECIMAL which limits range
• Using VARCHAR which wastes space and loses numeric features
• Using INT which cannot store decimals