CAST and CONVERT for type changes in SQL - Step-by-Step Execution

Concept Flow - CAST and CONVERT for type changes

Start with original value

↓

Choose target data type

↓

Apply CAST or CONVERT

↓

Check if conversion is valid

↓

Return converted value

↓

End

The flow shows how a value is taken, a target type is chosen, then CAST or CONVERT changes the type, checking validity before returning the result or error.

Execution Sample

SQL

SELECT CAST('123' AS INT) AS cast_result,
       CONVERT(VARCHAR, 456) AS convert_result;

This query converts a string '123' to an integer using CAST and converts an integer 456 to a string using CONVERT.

Execution Table

Step	Expression	Operation	Result	Notes
1	CAST('123' AS INT)	Convert string '123' to INT	123	String '123' is numeric, conversion succeeds
2	CONVERT(VARCHAR, 456)	Convert INT 456 to string	'456'	Integer 456 converted to string '456'
3	CAST('abc' AS INT)	Attempt to convert non-numeric string	Error or NULL	Conversion fails, returns error or NULL depending on SQL dialect
4	CONVERT(DATE, '2024-06-01')	Convert string to DATE	2024-06-01	Valid date string converted to DATE type
5	CAST(123.45 AS INT)	Convert float to INT	123	Decimal truncated to integer part
6	End	No more expressions	Query returns two columns with results	Execution complete

💡 All expressions processed; conversion errors handled by returning NULL or error depending on SQL dialect

Variable Tracker

Expression	Initial Value	After Conversion
CAST('123' AS INT)	'123' (string)	123 (int)
CONVERT(VARCHAR, 456)	456 (int)	'456' (string)
CAST('abc' AS INT)	'abc' (string)	Error or NULL
CONVERT(DATE, '2024-06-01')	'2024-06-01' (string)	2024-06-01 (date)
CAST(123.45 AS INT)	123.45 (float)	123 (int)

Key Moments - 3 Insights

Why does CAST('abc' AS INT) fail while CAST('123' AS INT) works?

Because 'abc' is not a numeric string, the conversion to INT is invalid and causes an error or NULL, as shown in execution_table step 3.

What happens when converting a float to an INT using CAST?

The decimal part is truncated, not rounded, so 123.45 becomes 123, as shown in execution_table step 5.

How does CONVERT differ from CAST in usage?

CONVERT allows specifying the target type first and can include style parameters for formatting; CAST uses a simpler syntax. Both perform type changes as shown in steps 1 and 2.

Visual Quiz - 3 Questions

Look at the execution_table, what is the result of CAST('123' AS INT) at step 1?

A '123'

B 123

C Error

D NULL

💡 Hint

Check the 'Result' column in execution_table row for step 1

At which step does the conversion fail due to invalid input?

A Step 4

B Step 2

C Step 3

D Step 5

💡 Hint

Look for 'Error or NULL' in the 'Result' column in execution_table

If we change CAST(123.45 AS INT) to CAST(123.99 AS INT), what would be the result?

A 123

B Error

C 124

D NULL

💡 Hint

Refer to execution_table step 5 showing truncation, not rounding

Concept Snapshot

CAST and CONVERT change data types in SQL.
CAST(expression AS target_type) is standard syntax.
CONVERT(target_type, expression) is similar, with optional style.
Invalid conversions cause errors or NULL.
Casting float to int truncates decimals.
Useful for data type compatibility and formatting.

Full Transcript

This lesson shows how SQL changes data types using CAST and CONVERT. We start with a value, choose a target type, then apply CAST or CONVERT. If the conversion is valid, the new value is returned; if not, an error or NULL occurs. For example, casting the string '123' to integer gives 123, but casting 'abc' to integer fails. Converting integer 456 to string results in '456'. Casting a float like 123.45 to integer truncates to 123. These functions help manage data types in queries.