Add SQL-style operators (#1322)

docs/content/3.manual/manual.yml

@@ -1835,6 +1835,46 @@ sections:
             input: '"2015-03-05T23:51:47Z"'
             output: ['1425599507']
 
+      - title: "SQL-Style Operators"
+        body: |
+
+          jq provides a few SQL-style operators.
+
+           * INDEX(stream; index_expression):
+
+             This builtin produces an object whose keys are computed by
+             the given index expression applied to each value from the
+             given stream.
+
+           * JOIN($idx; stream; idx_expr; join_expr):
+
+             This builtin joins the values from the given stream to the
+             given index.  The index's keys are computed by applying the
+             given index expression to each value from the given stream.
+             An array of the value in the stream and the corresponding
+             value from the index is fed to the given join expression to
+             produce each result.
+
+           * JOIN($idx; stream; idx_expr):
+
+             Same as `JOIN($idx; stream; idx_expr; .)`.
+
+           * JOIN($idx; idx_expr):
+
+             This builtin joins the input `.` to the given index, applying
+             the given index expression to `.` to compute the index key.
+             The join operation is as described above.
+
+           * IN(s):
+
+             This builtin outputs `true` if `.` appears in the given
+             stream, otherwise it outputs `false`.
+
+           * IN(source; s):
+
+             This builtin outputs `true` if any value in the source stream
+             appears in the second stream, otherwise it outputs `false`.
+
   - title: Conditionals and Comparisons
     entries:
       - title: "`==`, `!=`"