CAST / CONVERT

Overview

CAST / CONVERT — Change a Value from One Data Type to Another

Beginners usually need these when:

• A number is stored as text
• A date is stored as text
• You need to format a number
• You want to compare two columns with different data types

These functions help prevent errors and make data easier to work with.

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What CAST Does

CAST changes a value into a new data type.

CAST(value AS new_type)
  

Example idea:

• CAST('123' AS INT) → 123
• CAST(3.75 AS INT) → 3

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What CONVERT Does

CONVERT does the same thing as CAST, but uses a different format.

CONVERT(new_type, value)
  

Both are fine for beginners — they do the same job.

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Examples

Example 1: Convert credits (number) to text

SELECT course_name,
       CAST(credits AS CHAR) AS CreditsText
FROM courses;
  

Explanation

• Turns a number into text
• Useful when combining text and numbers in one column

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Example 2: Convert student_id (number) to text

SELECT first_name,
       last_name,
       CONVERT(CHAR, student_id) AS StudentIDText
FROM students;
  

Explanation

• Makes the ID a text value
• Helpful when joining with systems where IDs are stored as strings

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Example 3: Convert grade to a decimal

SELECT student_id,
       course_id,
       CAST(grade AS DECIMAL(5,2)) AS GradeDecimal
FROM enrollments;
  

Explanation

• Ensures grades always show two decimal places
• Useful for reports and calculations

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Example 4: Convert a text date into a real date

SELECT CAST('2024-09-01' AS DATE) AS CleanDate;
  

Explanation

• Turns a text string into a real date
• Allows use of date functions like DATEDIFF or DATEADD

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Why CAST / CONVERT Are Useful

• Fix data stored in the wrong data type
• Make comparisons accurate
• Prepare data for calculations
• Clean up messy imports

Beginners often hit data type mismatch errors — these functions solve that problem.

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CAST / CONVERT Summary

Function	Purpose	Beginner Explanation
CAST	Change a value to a new type	“Turn this into that.”
CONVERT	Same as CAST, different syntax	“Use this type for the value.”

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Practice Tasks (Your Turn!)

Task 1

Convert credits to text.

Task 2

Convert student_id to text.

Task 3

Convert grade to a decimal with two decimal places.