Minor: add test + docs for 2 argument trunc with columns (#7042)

* Minor: add test for 2 argument trunc with columns

* Update docs

* Add longer numbers to truncate

* add test for truncate with negative precision

* update docs with more details about precision

* prettier

datafusion/core/tests/sqllogictests/test_files/scalar.slt

@@ -918,6 +918,25 @@ select trunc(4.267, 3), trunc(1.1234, 2), trunc(-1.1231, 6), trunc(1.2837284, 2)
 ----
 4.267 1.12 -1.1231 1.28 1
 
+# trunc with negative precision should truncate digits left of decimal
+query R
+select trunc(12345.678, -3);
+----
+12000
+
+# trunc with columns and precision
+query RRR rowsort
+select
+  trunc(sqrt(abs(a)), 3) as a3,
+  trunc(sqrt(abs(a)), 1) as a1,
+  trunc(arrow_cast(sqrt(abs(a)), 'Float64'), 3) as a3_f64
+from small_floats;
+----
+0.447 0.4 0.447
+0.707 0.7 0.707
+0.837 0.8 0.837
+1 1 1
+
 ## bitwise and
 
 # bitwise and with column and scalar
@@ -1497,7 +1516,6 @@ true true false true true true
 # csv query boolean gt gt eq
 query BBBBBB rowsort
 SELECT a, b, a > b as gt, b = true as gt_scalar, a >= b as gt_eq, a >= true as gt_eq_scalar FROM t1
----- 
 ----
 NULL NULL NULL NULL NULL NULL
 NULL false NULL false NULL NULL
@@ -1512,10 +1530,10 @@ true true false true true true
 # csv query boolean distinct from
 query BBBBBB rowsort
 SELECT a, b,
-      a is distinct from b as df, 
-      b is distinct from true as df_scalar, 
-      a is not distinct from b as ndf, 
-      a is not distinct from true as ndf_scalar 
+      a is distinct from b as df,
+      b is distinct from true as df_scalar,
+      a is not distinct from b as ndf,
+      a is not distinct from true as ndf_scalar
 FROM t1
 ----
 NULL NULL false true true false

docs/source/user-guide/sql/scalar_functions.md

@@ -504,17 +504,23 @@ tanh(numeric_expression)
 
 ### `trunc`
 
-Truncates a number toward zero (at the decimal point).
+Truncates a number to a whole number or truncated to the specified decimal places.
 
 ```
-trunc(numeric_expression)
+trunc(numeric_expression[, decimal_places])
 ```
 
 #### Arguments
 
 - **numeric_expression**: Numeric expression to operate on.
   Can be a constant, column, or function, and any combination of arithmetic operators.
 
+- **decimal_places**: Optional. The number of decimal places to
+  truncate to. Defaults to 0 (truncate to a whole number). If
+  `decimal_places` is a positive integer, truncates digits to the
+  right of the decimal point. If `decimal_places` is a negative
+  integer, replaces digits to the left of the decimal point with `0`.
+
 ## Conditional Functions
 
 - [coalesce](#coalesce)