Add README with usage examples and implementation status

Author: azurechen97

README.md

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+# sqlglot-maxcompute
+
+A [SQLGlot](https://github.com/tobymao/sqlglot) dialect plugin for [Alibaba Cloud MaxCompute](https://www.alibabacloud.com/product/maxcompute) (formerly ODPS).
+
+Registers the `maxcompute` dialect via Python entry points so that SQLGlot can parse and generate MaxCompute SQL.
+
+## Installation
+
+```bash
+pip install sqlglot-maxcompute
+```
+
+## Usage
+
+```python
+import sqlglot
+
+# Parse MaxCompute SQL
+ast = sqlglot.parse_one("SELECT DATEADD(dt, 1, 'DAY')", read="maxcompute")
+
+# Transpile from another dialect to MaxCompute
+sqlglot.transpile(
+    "SELECT DATE_ADD(dt, 1)",
+    read="spark",
+    write="maxcompute",
+)
+# ["SELECT DATEADD(dt, 1, 'DAY')"]
+
+# Transpile from MaxCompute to another dialect
+sqlglot.transpile(
+    "SELECT DATETRUNC(dt, 'MONTH')",
+    read="maxcompute",
+    write="spark",
+)
+# ["SELECT TRUNC(dt, 'MONTH')"]
+
+# Round-trip: parse and regenerate MaxCompute SQL
+sqlglot.transpile(
+    "CREATE TABLE t (id INT) LIFECYCLE 30",
+    read="maxcompute",
+    write="maxcompute",
+)
+# ["CREATE TABLE t (id INT) LIFECYCLE 30"]
+```
+
+## What's implemented
+
+### Parser (MaxCompute → canonical AST)
+
+| Category | Functions |
+|---|---|
+| Date arithmetic | `DATEADD`, `DATEDIFF`, `ADD_MONTHS`, `MONTHS_BETWEEN` |
+| Date extraction | `DATEPART`, `DATETRUNC`, `TRUNC_TIME`, `DAYOFMONTH`, `DAYOFWEEK`, `DAYOFYEAR`, `HOUR`, `MINUTE`, `SECOND`, `QUARTER`, `WEEKDAY`, `WEEKOFYEAR` |
+| Date conversion | `DATE_FORMAT`, `TO_CHAR`, `TO_DATE`, `FROM_UNIXTIME`, `GETDATE`, `NOW`, `CURRENT_TIMESTAMP`, `CURRENT_TIMEZONE`, `FROM_UTC_TIMESTAMP` |
+| Last/next day | `LAST_DAY`, `LASTDAY`, `NEXT_DAY` |
+| String | `TOLOWER`, `TOUPPER`, `REGEXP_COUNT`, `SPLIT_PART` |
+| Aggregate | `WM_CONCAT`, `COUNT_IF`, `ARG_MAX`, `ARG_MIN`, `ANY_VALUE`, `APPROX_DISTINCT`, `STDDEV_SAMP`, `COVAR_POP`, `COVAR_SAMP`, `CORR`, `MEDIAN`, `PERCENTILE_APPROX`, `BITWISE_AND_AGG`, `BITWISE_OR_AGG`, `BITWISE_XOR_AGG` |
+| Array | `ALL_MATCH`, `ANY_MATCH`, `ARRAY_SORT`, `ARRAY_DISTINCT`, `ARRAY_EXCEPT`, `ARRAY_JOIN`, `ARRAY_MAX`, `ARRAY_MIN`, `ARRAYS_OVERLAP`, `ARRAYS_ZIP`, `ARRAY_INTERSECT`, `ARRAY_POSITION`, `ARRAY_REMOVE`, `ARRAY_CONTAINS` |
+| Map | `MAP_CONCAT`, `MAP_FROM_ENTRIES` |
+| JSON / misc | `FROM_JSON`, `GET_USER_ID`, `REGEXP_SUBSTR`, `SLICE`, `TO_MILLIS`, `ISDATE` |
+
+### Generator (canonical AST → MaxCompute SQL)
+
+- Date/time: `DATEADD`, `DATEDIFF`, `DATETRUNC`, `DATEPART`, `GETDATE()`, `NOW()`
+- String: `TOLOWER`, `TOUPPER`
+- Aggregate: `WM_CONCAT`, `ARG_MAX`, `ARG_MIN`, `APPROX_DISTINCT`
+- JSON/misc: `FROM_JSON`, `GET_USER_ID()`, `TO_MILLIS`, `TO_CHAR`
+- Type mapping: `VARCHAR`/`CHAR`/`TEXT` → `STRING`, `DATETIME` preserved
+
+### DDL
+
+- `LIFECYCLE n` — table retention in days
+- `RANGE CLUSTERED BY (cols) [SORTED BY (cols)] INTO n BUCKETS`
+- `AUTO PARTITIONED BY (TRUNC_TIME(col, 'unit') [AS alias])`
+- `TBLPROPERTIES ('key'='value')` coexists correctly with `LIFECYCLE`
+
+## Development
+
+```bash
+# Install dependencies
+uv sync
+
+# Run tests
+uv run pytest
+
+# Run a single test
+uv run pytest tests/test_maxcompute.py::TestMaxCompute::test_dateadd_roundtrip
+```
+
+## License
+
+MIT