Explicit Conversion
Explicit conversion is when you explicitly specify the conversion of data from one type to another using conversion functions. This type of conversion is controlled by the user and ensures that the data is transformed precisely as intended.
CAST Function
Syntax:
CAST(expression AS target_data_type)
Example:
SELECT CAST('2024-08-25' AS DATE) AS date_value;
In this example, the string ‘2024-08-25’ is explicitly converted to a DATE type.
CONVERT Function (SQL Server, MySQL)
Syntax:
CONVERT(target_data_type, expression [, style])
Example:
SELECT CONVERT(VARCHAR, GETDATE(), 1) AS date_string;
Here, GETDATE() returns the current date and time, and CONVERT changes it to a string in the format mm/dd/yy.
TO_CHAR, TO_DATE, TO_NUMBER (Oracle)
Syntax:
TO_CHAR(expression [, format])
TO_DATE(expression [, format])
TO_NUMBER(expression [, format])
Example:
SELECT TO_CHAR(SYSDATE, 'DD-MON-YYYY') AS date_string FROM dual;
This converts the current date into a string formatted as DD-MON-YYYY.
Implicit Conversion
Implicit conversion occurs automatically when you use different data types in expressions or comparisons, and SQL Server or another RDBMS converts them to a common type. This type of conversion happens without any explicit instruction from the user.
Automatic Type Promotion
SQL databases often automatically promote smaller data types to larger data types to ensure compatibility. For example, a TINYINT might be implicitly converted to an INT in calculations.
Example:
SELECT 5 + 3.2 AS result;
In this case, 5 (an INT) is implicitly converted to a FLOAT or DECIMAL to perform the addition with 3.2.
Comparing Different Data Types
When comparing different data types, SQL databases automatically convert them to a common type that can handle both values.
Example:
SELECT * FROM orders WHERE order_date = '2024-08-25';
If order_date is of type DATE and ‘2024-08-25’ is a string, SQL will implicitly convert the string to a ATE type to perform the comparison.
Inserting Data into a Table
When inserting data into a table, SQL will implicitly convert data if the source and target columns are of different types but compatible.
Example:
CREATE TABLE employees ( employee_id INT, salary DECIMAL(10, 2) ); INSERT INTO employees (employee_id, salary) VALUES (1, '50000.00');
Here, the string ‘50000.00’ is implicitly converted to a DECIMAL when inserted into the salary column.
Differences Between Explicit and Implicit Conversion
Control: Explicit conversion provides more control and clarity, as you specify the exact type conversion using functions. Implicit conversion happens automatically and might not always be obvious in your code.
Error Handling: Explicit conversion allows you to handle conversion errors more gracefully using functions like TRY_CAST or TRY_CONVERT in SQL Server. Implicit conversion may lead to unexpected results or errors if the data types are not compatible or if the implicit conversion is not what you intended.
Performance: While implicit conversions are convenient, they may lead to performance issues in large queries or when used excessively, as the database engine has to handle type conversions automatically. Explicit conversions, on the other hand, make your intentions clear and can sometimes be optimized better by the database engine.
Practical Examples
Explicit Conversion Example
Converting a string to a number and then performing a calculation:
SELECT CAST('123.45' AS DECIMAL(10, 2)) + 100 AS total_amount;
Implicit Conversion Example
Inserting and comparing different data types without explicit conversion:
— Implicit conversion during insertion
INSERT INTO employees (employee_id, salary) VALUES (2, 60000);
— Implicit conversion during comparison
SELECT * FROM employees WHERE salary = 60000;
Conclusion
Both explicit and implicit conversions play important roles in SQL. Explicit conversion gives you direct control over how data is transformed and ensures that data types are handled as expected. Implicit conversion provides convenience by automatically managing data type differences, but it may lead to unexpected behavior or performance issues if not properly understood.