Sql filter document (#228)

docs/src/sql.md

@@ -0,0 +1,96 @@
+# SQL filters
+
+LanceDB embraces the utilization of standard SQL expressions as predicates for hybrid
+filters. It can be used during hybrid vector search and deletion operations.
+
+Currently, Lance supports a growing list of expressions.
+
+* ``>``, ``>=``, ``<``, ``<=``, ``=``
+* ``AND``, ``OR``, ``NOT``
+* ``IS NULL``, ``IS NOT NULL``
+* ``IS TRUE``, ``IS NOT TRUE``, ``IS FALSE``, ``IS NOT FALSE``
+* ``IN``
+* ``LIKE``, ``NOT LIKE``
+* ``CAST``
+* ``regexp_match(column, pattern)``
+
+For example, the following filter string is acceptable:
+
+=== "Python"
+
+    ```python
+    tbl.search([100, 102])
+       .where("""(
+        (label IN [10, 20])
+        AND
+        (note.email IS NOT NULL)
+        ) OR NOT note.created
+        """)
+
+    ```
+=== "Javascript"
+
+    ```javascript
+    tbl.search([100, 102])
+       .where(`(
+            (label IN [10, 20])
+            AND
+            (note.email IS NOT NULL)
+        ) OR NOT note.created
+       `)
+    ```
+
+
+If your column name contains special characters or is a [SQL Keyword](https://docs.rs/sqlparser/latest/sqlparser/keywords/index.html),
+you can use backtick (`` ` ``) to escape it. For nested fields, each segment of the
+path must be wrapped in backticks.
+
+=== "SQL"
+    ```sql
+    `CUBE` = 10 AND `column name with space` IS NOT NULL
+      AND `nested with space`.`inner with space` < 2
+    ```
+
+!!! warning
+    Field names containing periods (``.``) are not supported.
+
+Literals for dates, timestamps, and decimals can be written by writing the string
+value after the type name. For example
+
+=== "SQL"
+    ```sql
+    date_col = date '2021-01-01'
+    and timestamp_col = timestamp '2021-01-01 00:00:00'
+    and decimal_col = decimal(8,3) '1.000'
+    ```
+
+For timestamp columns, the precision can be specified as a number in the type
+parameter. Microsecond precision (6) is the default.
+
+| SQL              | Time unit    |
+|------------------|--------------|
+| ``timestamp(0)`` | Seconds      |
+| ``timestamp(3)`` | Milliseconds |
+| ``timestamp(6)`` | Microseconds |
+| ``timestamp(9)`` | Nanoseconds  |
+
+LanceDB internally stores data in [Apache Arrow](https://arrow.apache.org/) format.
+The mapping from SQL types to Arrow types is:
+
+| SQL type | Arrow type |
+|----------|------------|
+| ``boolean`` | ``Boolean`` |
+| ``tinyint`` / ``tinyint unsigned`` | ``Int8`` / ``UInt8`` |
+| ``smallint`` / ``smallint unsigned`` | ``Int16`` / ``UInt16`` |
+| ``int`` or ``integer`` / ``int unsigned`` or ``integer unsigned`` | ``Int32`` / ``UInt32`` |
+| ``bigint`` / ``bigint unsigned`` | ``Int64`` / ``UInt64`` |
+| ``float`` | ``Float32`` |
+| ``double`` | ``Float64`` |
+| ``decimal(precision, scale)`` | ``Decimal128`` |
+| ``date`` | ``Date32`` |
+| ``timestamp`` | ``Timestamp`` [^1] |
+| ``string`` | ``Utf8`` |
+| ``binary`` | ``Binary`` |
+
+[^1]: See precision mapping in previous table.
+