CAST and CONVERT for type changes in SQL

Introduction

We use CAST and CONVERT to change data from one type to another, like turning text into numbers or dates. This helps us work with data in the right way.

When you have a number stored as text and want to do math with it.

When you want to show a date in a different format.

When combining data from different tables with different data types.

When you need to compare values but their types don't match.

When importing data that is not in the correct type and needs conversion.

Syntax

SQL

CAST(expression AS target_data_type)
CONVERT(target_data_type, expression [, style])

CAST is standard SQL and works in most databases.

CONVERT is often used in SQL Server and allows an optional style for formatting.

Examples

This changes the text '123' into the number 123.

SQL

SELECT CAST('123' AS INT);

This converts the current date to a string in 'YYYY-MM-DD' format.

SQL

SELECT CONVERT(VARCHAR, GETDATE(), 23);

This converts the decimal number 123.45 to the integer 123 by dropping the decimal part.

SQL

SELECT CAST(123.45 AS INT);

Sample Program

This query converts a text date to a date type, then converts it back to a string in MM/DD/YYYY format.

SQL

SELECT CAST('2024-06-01' AS DATE) AS ConvertedDate,
       CONVERT(VARCHAR, CAST('2024-06-01' AS DATE), 101) AS FormattedDate;

Important Notes

CAST is more portable across different SQL databases.

CONVERT's style parameter is useful for date and time formatting in SQL Server.

Be careful converting types that don't match well, like text that isn't a number.

Summary

CAST and CONVERT change data types to help with calculations and formatting.

CAST is standard SQL; CONVERT has extra formatting options in some databases.

Use them to fix data type mismatches and display data nicely.

Practice

1. What is the main purpose of using CAST or CONVERT in SQL?

easy

A. To change the data type of a value

B. To delete rows from a table

C. To create a new table

D. To join two tables

Solution

1. Step 1: Understand the function of CAST and CONVERT

   Both CAST and CONVERT are used to change the data type of a value in SQL.

2. Step 2: Compare with other options

   Deleting rows, creating tables, and joining tables are unrelated to changing data types.

3. Final Answer:

   To change the data type of a value -> Option A

4. Quick Check:

   CAST and CONVERT change types = A [OK]

Hint: CAST and CONVERT always change data types [OK]

Common Mistakes:

• Thinking CAST deletes data
• Confusing CONVERT with table creation
• Mixing data type change with joins

2. Which of the following is the correct syntax to convert a string '123' to an integer using CAST in SQL?

easy

A. CAST('123' AS INT)

B. CAST(INT AS '123')

C. CONVERT('123', INT)

D. CONVERT(INT, '123')

Solution

1. Step 1: Recall CAST syntax

   CAST requires the format CAST(value AS datatype), so CAST('123' AS INT) is correct.

2. Step 2: Check other options

   Options A, B, and D use incorrect order or syntax for CAST or CONVERT.

3. Final Answer:

   CAST('123' AS INT) -> Option A

4. Quick Check:

   CAST(value AS type) = C [OK]

Hint: CAST syntax is always CAST(value AS datatype) [OK]

Common Mistakes:

• Swapping value and datatype order
• Using CONVERT with wrong argument order
• Missing AS keyword in CAST

3. Given the SQL query:

SELECT CAST('2024-06-15' AS DATE) AS ConvertedDate;

What will be the output type of ConvertedDate?

medium

A. String

B. Boolean

C. Integer

D. Date

Solution

1. Step 1: Analyze the CAST operation

   The query casts the string '2024-06-15' to the DATE data type.

2. Step 2: Determine the output type

   Since CAST converts to DATE, the output column ConvertedDate will be of type Date.

3. Final Answer:

   Date -> Option D

4. Quick Check:

   CAST to DATE returns Date type = D [OK]

Hint: CAST to DATE returns a date type value [OK]

Common Mistakes:

• Assuming output stays string
• Confusing DATE with integer
• Thinking CAST changes value format only

4. Identify the error in this SQL statement:

SELECT CONVERT(INT, 'abc123');

medium

A. No error, converts string to integer

B. Syntax error due to wrong CONVERT argument order

C. Runtime error converting non-numeric string to INT

D. Missing AS keyword in CONVERT

Solution

1. Step 1: Check CONVERT syntax

   CONVERT(INT, 'abc123') is correct syntax in some SQL dialects like SQL Server.

2. Step 2: Consider conversion feasibility

   Trying to convert 'abc123' to INT causes a runtime error because 'abc123' is not numeric.

3. Final Answer:

   Runtime error converting non-numeric string to INT -> Option C

4. Quick Check:

   Non-numeric string to INT causes runtime error = A [OK]

Hint: Non-numeric strings cause errors when converting to INT [OK]

Common Mistakes:

• Assuming CONVERT always succeeds
• Confusing syntax error with runtime error
• Expecting implicit string to int conversion

5. You have a table Orders with a column OrderDate stored as VARCHAR in format 'YYYYMMDD'.
Which SQL query correctly converts OrderDate to a DATE type and selects orders from June 2024?

Options:
A) SELECT * FROM Orders WHERE CAST(OrderDate DATE) BETWEEN '2024-06-01' AND '2024-06-30';
B) SELECT * FROM Orders WHERE CONVERT(DATE, OrderDate, 112) BETWEEN '2024-06-01' AND '2024-06-30';
C) SELECT * FROM Orders WHERE CAST(OrderDate AS INT) BETWEEN 20240601 AND 20240630;
D) SELECT * FROM Orders WHERE CONVERT(INT, OrderDate) BETWEEN 20240601 AND 20240630;

hard

A. Option A only

B. Option B only

C. Option C only

D. Option D only

Solution

1. Step 1: Understand the data format and conversion

   OrderDate is VARCHAR in 'YYYYMMDD' format. CONVERT with style 112 converts this string correctly to DATE.

2. Step 2: Evaluate each option

   Option B only only tries CAST without style, which may fail or misinterpret format. Option B only only uses CONVERT with style 112, correct for 'YYYYMMDD'. Options C and D convert to INT, which is not reliable for date comparisons.

3. Final Answer:

   Option B only -> Option B

4. Quick Check:

   CONVERT with style 112 converts 'YYYYMMDD' to DATE = B [OK]

Hint: Use CONVERT with style 112 for 'YYYYMMDD' to DATE [OK]

Common Mistakes:

• Using CAST without format for 'YYYYMMDD'
• Converting dates to INT for comparison
• Ignoring CONVERT style parameter