Read carefully examples in DOCS here:
https://reference.wolfram.com/language/ref/Dataset.html
Create a Dataset object from tabular data:
In[]:=
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"->{}|>}]
Out[]=
a
b
c
1
x
{1}
2
y
{2,3}
3
z
{3}
4
x
{4,5}
5
y
{5,6,7}
6
z
{}
Construct a new table by specifying operators that will compute each column:
In[]:=
dataset[All,Key["c"]/*<|"ctotal"->Total,"clength"->Length|>]
Out[]=
ctotal
clength
1
1
5
2
3
1
9
2
18
3
0
0
Use the same technique to rename columns:
In[]:=
dataset[All,<|"A"->"a","B"->"b","C"->"c"|>]
Out[]=
A
B
C
1
x
{1}
2
y
{2,3}
3
z
{3}
4
x
{4,5}
5
y
{5,6,7}
6
z
{}
In your case it becomes:
In[]:=
dataset=Dataset[{{195,2.3},{198,6.4},{204,-3.1}}]
Out[]=
195
2.3
198
6.4
204
-3.1
In[]:=
dataset[All,<|"x"->#[[1]],"y"->#[[2]]|>&]
Out[]=
x
y
195
2.3
198
6.4
204
-3.1