Dynamic attrs

We can use attrs for other things as well.

Dynamic projections

For example, projecting from a dynamically generated list of attributes. If we want to project the attributes of S that are of type int, say, then we could:

S{attrs(heading(S)[type="int"])}

STATUS

10
20
30

Or if we want to project the attributes of S that are not of type int then we could:

S~{attrs(heading(S)[type="int"])}

SNO
SNAME
CITY

S1
Smith
London
S2
Jones
Paris
S3
Blake
Paris
S4
Clark
London
S5
Adams
Athens

Dynamic variables

We can also pass attrs to schema, which lets us select variables from the current schema (schema giving us a single tuple containing an attr/value for each entry in the schema). If we want to get the variable named S, say, then we could:

schema{attrs(dee(attr:="S"))}.*

SNO
SNAME
STATUS
CITY

S1
Smith
20
London
S2
Jones
10
Paris
S3
Blake
30
Paris
S4
Clark
20
London
S5
Adams
30
Athens

Where you'll notice the S we selected was passed as a string and evaluated by attrs at runtime.

Dynamic everything

And of course we can have a dynamic projection on a dynamically chosen relvar:

schema{attrs(dee(attr:="S"))}.*{attrs(heading(schema{attrs(dee(attr:="S"))}.*)[type="int"])}

STATUS

10
20
30