What is the reason the two datasets below would be unable to be merged together? $($Note: the variable county id is highlighted in red in the first dataset, while it is highlighted in black in the second$)$

$\backslash$table$[[,$county$\_1$d$,$average$\_$rent,,county$\_$id$,$popdensity$],[1,,1931\mathrm{.}035,1,1,266\mathrm{.}5309],[2,,1454\mathrm{.}888,2,?,362\mathrm{.}79=3$