Every qSQL query has an equivalent functional form in fact, is parsed into it, so there is no difference in their execution. The qSQL queries look like SQL and we use them wherever possible, but the functional forms are more general, and some more complex queries need them.
The q language adds those SQL-like query templates to the k4 language, and loosely speaking their functional equivalents are the k4 forms. When you write a functional query in q, you are coding in k.
In the latest episode of the Array Cast podcast we interview Michal Wallace (@tangentstorm) of 1010data, who became a k3 dev and liked it. Hes not swivel-eyed about it. You could hire him to work in a shop that doesnt use k youd just have to double his salary.
Listen to the podcast to learn why.