DESCRIBE 语句

本文档是 Apache Flink 的旧版本。建议访问最新的稳定版本。

执行 DESCRIBE 语句
语法

DESCRIBE 语句用来描述一张表或者视图的 Schema。

执行 DESCRIBE 语句

DESCRIBE 语句可以通过 TableEnvironment 的 executeSql() 执行，也可以在 SQL CLI 中执行 DROP 语句。若 DESCRIBE 操作执行成功，executeSql() 方法返回该表的 Schema，否则会抛出异常。

以下的例子展示了如何在 TableEnvironment 和 SQL CLI 中执行一个 DESCRIBE 语句。

EnvironmentSettings settings = EnvironmentSettings.newInstance()...
TableEnvironment tableEnv = TableEnvironment.create(settings);

// register a table named "Orders"
tableEnv.executeSql(
        "CREATE TABLE Orders (" +
        " `user` BIGINT NOT NULl," +
        " product VARCHAR(32)," +
        " amount INT," +
        " ts TIMESTAMP(3)," +
        " ptime AS PROCTIME()," +
        " PRIMARY KEY(`user`) NOT ENFORCED," +
        " WATERMARK FOR ts AS ts - INTERVAL '1' SECONDS" +
        ") with (...)");

// print the schema
tableEnv.executeSql("DESCRIBE Orders").print();

val settings = EnvironmentSettings.newInstance()...
val tableEnv = TableEnvironment.create(settings)

// register a table named "Orders"
 tableEnv.executeSql(
        "CREATE TABLE Orders (" +
        " `user` BIGINT NOT NULl," +
        " product VARCHAR(32)," +
        " amount INT," +
        " ts TIMESTAMP(3)," +
        " ptime AS PROCTIME()," +
        " PRIMARY KEY(`user`) NOT ENFORCED," +
        " WATERMARK FOR ts AS ts - INTERVAL '1' SECONDS" +
        ") with (...)")

// print the schema
tableEnv.executeSql("DESCRIBE Orders").print()

settings = EnvironmentSettings.new_instance()...
table_env = StreamTableEnvironment.create(env, settings)

# register a table named "Orders"
table_env.execute_sql( \
        "CREATE TABLE Orders (" 
        " `user` BIGINT NOT NULl," 
        " product VARCHAR(32),"
        " amount INT,"
        " ts TIMESTAMP(3),"
        " ptime AS PROCTIME(),"
        " PRIMARY KEY(`user`) NOT ENFORCED,"
        " WATERMARK FOR ts AS ts - INTERVAL '1' SECONDS"
        ") with (...)");

# print the schema
table_env.execute_sql("DESCRIBE Orders").print()

Flink SQL> CREATE TABLE Orders (
>  `user` BIGINT NOT NULl,
>  product VARCHAR(32),
>  amount INT,
>  ts TIMESTAMP(3),
>  ptime AS PROCTIME(),
>  PRIMARY KEY(`user`) NOT ENFORCED,
>  WATERMARK FOR ts AS ts - INTERVAL '1' SECONDS
> ) with (
>  ...
> );
[INFO] Table has been created.

Flink SQL> DESCRIBE Orders;

上述例子执行的结果为:

+---------+----------------------------------+-------+-----------+-----------------+----------------------------+
|    name |                             type |  null |       key | computed column |                  watermark |
+---------+----------------------------------+-------+-----------+-----------------+----------------------------+
|    user |                           BIGINT | false | PRI(user) |                 |                            |
| product |                      VARCHAR(32) |  true |           |                 |                            |
|  amount |                              INT |  true |           |                 |                            |
|      ts |           TIMESTAMP(3) *ROWTIME* |  true |           |                 | `ts` - INTERVAL '1' SECOND |
|   ptime | TIMESTAMP(3) NOT NULL *PROCTIME* | false |           |      PROCTIME() |                            |
+---------+----------------------------------+-------+-----------+-----------------+----------------------------+
5 rows in set

+---------+----------------------------------+-------+-----------+-----------------+----------------------------+
|    name |                             type |  null |       key | computed column |                  watermark |
+---------+----------------------------------+-------+-----------+-----------------+----------------------------+
|    user |                           BIGINT | false | PRI(user) |                 |                            |
| product |                      VARCHAR(32) |  true |           |                 |                            |
|  amount |                              INT |  true |           |                 |                            |
|      ts |           TIMESTAMP(3) *ROWTIME* |  true |           |                 | `ts` - INTERVAL '1' SECOND |
|   ptime | TIMESTAMP(3) NOT NULL *PROCTIME* | false |           |      PROCTIME() |                            |
+---------+----------------------------------+-------+-----------+-----------------+----------------------------+
5 rows in set

+---------+----------------------------------+-------+-----------+-----------------+----------------------------+
|    name |                             type |  null |       key | computed column |                  watermark |
+---------+----------------------------------+-------+-----------+-----------------+----------------------------+
|    user |                           BIGINT | false | PRI(user) |                 |                            |
| product |                      VARCHAR(32) |  true |           |                 |                            |
|  amount |                              INT |  true |           |                 |                            |
|      ts |           TIMESTAMP(3) *ROWTIME* |  true |           |                 | `ts` - INTERVAL '1' SECOND |
|   ptime | TIMESTAMP(3) NOT NULL *PROCTIME* | false |           |      PROCTIME() |                            |
+---------+----------------------------------+-------+-----------+-----------------+----------------------------+
5 rows in set

root
 |-- user: BIGINT NOT NULL
 |-- product: VARCHAR(32)
 |-- amount: INT
 |-- ts: TIMESTAMP(3) *ROWTIME*
 |-- ptime: TIMESTAMP(3) NOT NULL *PROCTIME* AS PROCTIME()
 |-- WATERMARK FOR ts AS `ts` - INTERVAL '1' SECOND
 |-- CONSTRAINT PK_3599338 PRIMARY KEY (user)

语法

DESCRIBE [catalog_name.][db_name.]table_name