CAST and CONVERT for type changes in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the CAST function do in SQL?

CAST changes a value from one data type to another in SQL. For example, it can turn a number into text or a date into a string.

intermediate

How is CONVERT different from CAST in SQL?

CONVERT also changes data types but often allows formatting options, especially for dates. CAST is more standard and simpler.

beginner

Write a SQL example using CAST to convert an integer 123 to a string.

SELECT CAST(123 AS VARCHAR(10));

intermediate

Write a SQL example using CONVERT to change a date to a string with format style 101 (mm/dd/yyyy).

SELECT CONVERT(VARCHAR(10), GETDATE(), 101);

beginner

Why might you use CAST or CONVERT in a SQL query?

To make sure data types match when comparing or combining data, or to format data for display or calculations.

Which SQL function is more standard across different database systems for type conversion?

A CONVERT

B FORMAT

C CAST

D TO_CHAR

CAST is part of the SQL standard and works in most databases, while CONVERT is often specific to certain systems like SQL Server.

What does this SQL do? SELECT CAST('2024-06-01' AS DATE);

A Converts a string to a date

B Converts a date to a string

C Changes a number to a string

D Changes a string to a number

CAST here changes the text '2024-06-01' into a date type.

Which CONVERT style code formats a date as mm/dd/yyyy in SQL Server?

A 120

B 101

C 103

D 1

Style 101 formats dates as mm/dd/yyyy in SQL Server's CONVERT function.

If you want to convert an integer to a string in SQL, which is correct?

A CAST(123 AS VARCHAR)

B CONVERT(INT, '123')

C CAST('123' AS INT)

D CONVERT(DATE, 123)

CAST(123 AS VARCHAR) converts the number 123 to a string.

Which function allows you to specify a style or format when converting types?

A SUBSTRING

B CAST

C LEN

D CONVERT

CONVERT lets you specify a style code for formatting, especially useful for dates.

Explain how CAST and CONVERT are used to change data types in SQL and when you might choose one over the other.

Think about standard use versus formatting needs.

Describe a real-life example where converting data types in SQL is necessary.

Imagine working with a report or data from different sources.

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