Newbie Question about slip joints.

I have seen some p traps that have a threaded swivel 90 attached to them, obviously it works,
but wont standing water leak through after some time?

I also saw this picture in another thread and it looks like the slip joint is at the same level or slightly below the 90 after the p trap is that code or prone to leaking?

One thing to remember is that when plumbed properly, none of that piping holds water with the exception of the wier of the trap. A slip joint that is properly tightened will not leak.

Their disadvantage is that they do not make for a solid installation, whereas if they get bumped around the piping might move and the fitting could become loose.

On the 'swivel joint' of that type of trap, there is an integral seal...meaning a bevel on one piece, and a corresponding groove on the other. When tightened together, this is called a "metal to metal" seal, although of course in this case it it plastic to plastic. It is very reliable, rarely have any leak issues. This type of seal , in metal, can be used even in pressure fittings.