add docstring for SQLVisitor

src/utils/sql.jl

@@ -1,5 +1,3 @@
-"""
-"""
 module SQL
 
 import ..ASTWalker: Visitor, walk, before, visit, after
@@ -52,6 +50,23 @@ const SQL_OP_MAP = Dict{String,String}(
 
 const VALID_SQL_OPS = Set(keys(SQL_OP_MAP))
 
+"""
+ SQLVisitor
+
+Visitor that converts an OPA partial compile AST to a SQL condition.
+
+It requires two dictionaries to be passed in the constructor:
+- `schema_map`: maps OPA package names to database schema names
+- `table_map`: maps OPA rule names to database table names
+
+Input to the visitor must be a partial compile result from OPA already converted to a julia representation using `ASTWalker.AST.ASTVisitor`.
+Walking the AST using this visitor will result in a SQL condition that can be appended to a SQL query using a `where` clause.
+Output, that is returned from the `walk` method, is an `AbstractSQLCondition`. It can be one of:
+
+- `SQLCondition`: represents a SQL condition. Contains the SQL string that represents the condition that can be used in the query with a "where" clause.
+- `UnconditionalInclude`: represents an unconditional include condition. Which means that the SQL query should return all rows.
+- `UnconditionalExclude`: represents an unconditional exclude condition. Which means that the SQL query should not return any rows.
+"""
 struct SQLVisitor <: Visitor
  schema_map::Dict{String, String}
  table_map::Dict{String, String}