  We obtain new and precise information on the double white dwarf (DWD)
population and on its gravitational-wave-driven merger rate, by combining the
constraints on the DWD population from two previous radial-velocity-variation
studies: One based on a sample of white dwarfs (WDs) from the Sloan Digital Sky
Survey (SDSS, which with its low spectral resolution probes systems at
separations a<0.05 au), and one based on the ESO-VLT Supernova-Ia Progenitor
surveY (SPY, which, with high spectral resolution, is sensitive to a<4 au).
From a joint likelihood analysis, the DWD fraction among WDs is
fbin=0.095+/-0.020 (1-sigma, random) +0.010 (systematic) in the separation
range ~<4 au. The index of a power-law distribution of initial WD separations
(at the start of solely gravitational-wave-driven binary evolution), N(a)da ~
a^alpha da, is alpha=-1.30+/-0.15 (1-sigma) +0.05 (systematic). The Galactic WD
merger rate per WD is R_merge=(9.7+/-1.1)e-12 /yr. Integrated over the Galaxy
lifetime, this implies that 8.5-11 per cent of all WDs ever formed have merged
with another WD. If most DWD mergers end as more-massive WDs, then some 10 per
cent of WDs are DWD-merger products, consistent with the observed fraction of
WDs in a "high-mass bump" in the WD mass function. The DWD merger rate is 4.5-7
times the Milky Way's specific Type-Ia supernova (SN Ia) rate. If most SN Ia
explosions stem from the mergers of some DWDs (say, those with massive-enough
binary components) then ~15 per cent of all WD mergers must lead to a SN Ia.
