Variables — `let` & `into`

Variables assign a name — say x — to an expression, like in most programming languages. The name can then be used in any expression, acting as a substitute for the expression x.

Syntactically, variables can take 3 forms.

let declares the name before the expression.
prql
```
let my_name = x
```
into declares the name after the expression. This form is useful for quick pipeline splitting and conforms with the "flow from top to bottom" rule of pipelines.
prql
```
x
into my_name
```
The final expression of a pipeline defaults to taking the name main.
prql
```
from x
```
... is equivalent to:
prql
```
let main = x
```

When compiling to SQL, relational variables are compiled to Common Table Expressions (or sub-queries in some cases).

prql

let top_50 = (
  from employees
  sort salary
  take 50
  aggregate {total_salary = sum salary}
)

from top_50      # Starts a new pipeline

prql

from employees
take 50
into first_50

from first_50

Variables can be assigned an s-string containing the whole SQL query s-string, enabling us to use features which PRQL doesn't yet support.

prql

let grouping = s"""
  SELECT SUM(a)
  FROM tbl
  GROUP BY
    GROUPING SETS
    ((b, c, d), (d), (b, d))
"""

from grouping

Variables — `let` & `into`

Variables — let & into

Variables — `let` & `into`