Dataset[\[LeftAssociation]a->\[LeftAssociation]x->7\[RightAssociation]\[RightAssociation]]

ds=Dataset[\[LeftAssociation]"a"->\[LeftAssociation]"x"->7,"y"->9\[RightAssociation], "b"->\[LeftAssociation]"x"->7,"y"->9\[RightAssociation]\[RightAssociation]]

ds[["a"]]

ds[[All,"y"]]

ds["a"]

ds[All,"y"]

Dataset[{{7}}]

dataset=Dataset[{
<|"a"->1,"b"->"x","c"->{1}|>,
<|"a"->2,"b"->"y","c"->{2,3}|>,
<|"a"->3,"b"->"z","c"->{3}|>,
<|"a"->4,"b"->"x","c"->{4,5}|>,
<|"a"->5,"b"->"y","c"->{5,6,7}|>,
<|"a"->6,"b"->"z","c"->{}|>}]

ds=Dataset[
Association@Table[
(Subscript[t, i])->Association@Table[
(Subscript[h, j])->ij,
{j,1,3}
],
{i,1,4}
]
]

ds[[(Subscript[t, 1])]]

ds[(Subscript[t, 1])]