where

The keyword where generally denotes a table being filtered in Envision.

where (cond), where block

In a where block, the keyword where filters the specified table.

table T = with
  [| as N |]
  [| 0 |]
  [| 1 |]
  [| 2 |]
  [| 3 |]

where T.N mod 2 == 0 // where block
  // 2-space indent required inside the block
  T.Twice = 2 * T.N
  show table "filtered" a1b2 with T.N, T.Twice

The Scalar table cannot be filtered by where. Attempting to filter the Scalar table will fail at compile-time.

A where block cannot appear inside a show statement. Unlike SQL statements where the WHERE keyword appers after the SELECT keyword, in Envision, where appears before the show statement.

The where block is typically intended to keep the same filter(s) active while several operations happen.

The indent can be avoided by using the keyword keep:

table T = with
  [| as N |]
  [| 0 |]
  [| 1 |]
  [| 2 |]
  [| 3 |]

keep where T.N mod 2 == 0
// we are inside the 'where' block, but no indent due to 'keep'
T.Twice = 2 * T.N
show table "filtered" a1b2 with T.N, T.Twice

.. = (expr) where (cond), filtered assignment

On the right side of an assignment, the keyword where filters the line being effectively assigned.

table Products = with // The table 'Products' has 3 lines
  [| as Product, as Price |]
  [| "cap",      3.50     |]
  [| "hat",      2.50     |]
  [| "ball",     4.00     |]

// This assignment is filtered by 'Products.Price < 3'
// Only the 'hat' line is modified
Products.Price = Products.Price * 1.1 where Products.Price < 3

// The 'hat' line has now a price of '2.75'
show table "" with Products.Product, Products.Price

This syntax is logically equivalent to the more verbose where block:

table Products = with
  [| as Product, as Price |]
  [| "cap",      3.50     |]
  [| "hat",      2.50     |]
  [| "ball",     4.00     |]

where Products.Price < 3
  Products.Price = Products.Price * 1.1 

show table "" with Products.Product, Products.Price

table .. = where (cond), filtered table creation

In a table definition, the keyword where indicates the creation of a filtered table.

table Orders[odim] = with
  [| as MyId, as MyDate, as Quantity |]
  [| "A",  date(2023,1,5), 5 |]
  [| "A",  date(2023,1,11), 2 |]
  [| "B",  date(2023,1,20), 1 |]

// The table 'OnlyA' is filtered from 'Orders'.
// Original columns of 'Orders' are cloned into 'OnlyA'.
table OnlyA[oadim] = where Orders.MyId == "A" 

// Setup the relation betwen tables 'Orders' and 'OnlyA'
expect OnlyA.odim = OnlyA.oadim

// Broadcast from 'Orders' to 'OnlyA' is now possible
OnlyA.Twice = 2 * Orders.Quantity

show table "A only" with OnlyA.MyId, OnlyA.MyDate, OnlyA.Twice