Power Query Structured lookup

[Dicken]

Hi, 
Can someone help or at least point me in direction of  information on a structured lookup; 

 example    position operator   table  { 0 } ,  so the starting point is  [ A = 1 = [A = 2]] = a true / false , 
 so I have a fact table ; 

Table.FromRecords(
{ [Item = "A"], [Item = "B"], [Item = "C"],[Item = "D"],
[Item = "A"] , [Item = "C"] , [Item = "C"] , [Item = "D"] } )

and a lookup 

Table.FromRecords( { [Item = "A", Unit = 2],
[Item = "B", Unit = 3],
[Item = "C", Unit = 4],
[Item = "D", Unit = 5]} )

 so I can lookup a  value ; 

Table.AddColumn( Fact, "N", each  lookup { [Item = [Item]]  )) 

This returns the correct corresponding row as a record ; but I can't get to grips with how it works, as it is I presume returnng a true/false as a list? not sure, 

This is the code for what I have as example ; 

= [
lookupT = Table.FromRecords( { [Item = "A", Unit = 2],
[Item = "B", Unit = 3],
[Item = "C", Unit = 4],
[Item = "D", Unit = 5]} ),

Fact = Table.FromRecords(
{ [Item = "A"], [Item = "B"], [Item = "C"],[Item = "D"],
[Item = "A"] , [Item = "C"] , [Item = "C"] , [Item = "D"] } ) ,
Addcol = Table.AddColumn( Fact, "N", each
lookupT { [Item = [Item]]} )]

Richard. 

[ZhangKun]

This is a native syntax that means find the only row in the table that meets the conditions.

You can find its usage in the Power Query documentation.

https://learn.microsoft.com/en-us/powerquery-m/m-spec-operators#selection-and-projection-operators

For example, tbl{[A=1, B=2]) means to find the row in table tbl where column A is equal to 1 and column B is equal to 2 (return a record value).

You may know that the operator ? can prevent errors when an item or field does not exist, but it may not work here. The operator ? can only avoid not being found. If multiple rows that meet the conditions are found, an error will still be returned.

// null
Table.FromValue({1, 2, 3, 3}){[Value = 4]}?

// error
Table.FromValue({1, 2, 3, 3}){[Value = 3]}?

[Dicken]

just to add having had  al look , I knew using  Table [ [ Column] ]   was projection, but would not have thought to search for that so i've learned something already. 

[ZhangKun · Accepted solution]

There are some more examples that may confuse you. If you can understand them, then you understand "field access" (and projection) and the keyword each.

// Example 1
[A = 1, B = 2][[A], [C]]?

// Example 2
#table({"A", "B"}, {{1, 2}, {1, 3}})[[D]]?

// Example 3
let 
    _ = [
        A = 1, 
        B = 2
    ]
in 
    [A]

// Example 4
let 
    _ = [
        A = 1, 
        B = 2
    ]
in 
    _[[A], [C]]?

[Dicken]

Thanks I'll have a look, 
   I'll leave this open for a bit  and see if anyone else has reading suggestions. 

Richad.