docs: language design notes (#1830)

book/src/internals/language-design.md

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+# Language design
+
+In a way PRQL is just a transpiler to SQL. This can cause its language design to
+gravitate toward thinking about PRQL features in terms of how they translate to
+SQL.
+
+```
+PRQL feature -> SQL feature -> relational result
+```
+
+This is flawed because:
+
+- it does not model interactions between features well,
+- SQL behavior can sometimes be misleading (the order of a subquery will not
+  persist in the parent query) or even differs between dialects (set
+  operations).
+
+Instead, we should think of PRQL features in terms of how they affect PRQL
+expressions, which in most cases means how they affect relations.
+
+```
+PRQL feature -> relation
+                   |
+                   v
+PRQL feature -> relation
+                   |
+                   v
+PRQL feature -> relation
+                   |
+                   v
+            relational result
+```
+
+Thinking about SQL comes in only at the last step when relation (or rather
+relational expression) is translated to an SQL expression.