[Hamuko]

Some say that Microsoft Excel is the world's most popular database engine.

[bastawhiz]

Excel allows you to perform queries for almost arbitrary data on a spreadsheet. I wouldn't doubt for a minute that with sufficient time and motivation you could write a full sql engine with excel functions.

[tomnipotent]

Excel has had a robust built-in columnar database along with decent query semantics and data modeling for awhile now (Power Query/Power Pivot).

[Baeocystin]

Honestly, I'd agree with that, too, at least in a colloquial sense.

[victor106]

The most popular database where the number of concurrent users = 1

[CyberDildonics]

Depends on how many people are looking over your shoulder.