Thank you for the great explanation. But SQL isn't as dynamically typed as you suggest. If a column is defined as DECIMAL(8, 2), it would be surprising for some values in that column to be strings. RDBMSs are expected to provide data integrity guarantees, and one of those guarantees is that only values matching the declared column type can be stored.

Relaxing that guarantee has benefits. For example, it can make application evolution easier--being able to store strings in a column originally intended for numbers is convenient. But that convenience can become a liability when multiple applications read from and write to the same database. In those cases, you want applications to adhere to a shared schema contract, and the RDBMS is typically expected to enforce that contract.

It also creates problems for generic tools such as reporting systems, which rely on stable data types--for example, to determine whether a column can be aggregated or how it should be formatted for display.