系统（内置）函数 #

返回给定时间列属性 rowtime 的当前水印，如果管道中的当前操作没有可用的上游操作的公共水印时则为 NULL。 函数的返回类型被推断为与提供的时间列属性匹配，但调整后的精度为 3。例如时间列属性为 TIMESTAMP_LTZ(9)，则函数将返回 TIMESTAMP_LTZ(3)。

请注意，此函数可以返回 NULL，您可能必须考虑这种情况。例如，如果您想过滤掉后期数据，您可以使用：

WHERE
  CURRENT_WATERMARK(ts) IS NULL
  OR ts > CURRENT_WATERMARK(ts)

Determines whether a JSON string satisfies a given path search criterion.

If the error behavior is omitted, FALSE ON ERROR is assumed as the default.

// TRUE
SELECT JSON_EXISTS('{"a": true}', '$.a');
// FALSE
SELECT JSON_EXISTS('{"a": true}', '$.b');
// TRUE
SELECT JSON_EXISTS('{"a": [{ "b": 1 }]}',
  '$.a[0].b');

// TRUE
SELECT JSON_EXISTS('{"a": true}',
  'strict $.b' TRUE ON ERROR);
// FALSE
SELECT JSON_EXISTS('{"a": true}',
  'strict $.b' FALSE ON ERROR);

Extracts a scalar from a JSON string.

This method searches a JSON string for a given path expression and returns the value if the value at that path is scalar. Non-scalar values cannot be returned. By default, the value is returned as STRING. Using returningType a different type can be chosen, with the following types being supported:

• VARCHAR / STRING
• BOOLEAN
• INTEGER
• DOUBLE

For empty path expressions or errors a behavior can be defined to either return null, raise an error or return a defined default value instead. When omitted, the default is NULL ON EMPTY or NULL ON ERROR, respectively. The default value may be a literal or an expression. If the default value itself raises an error, it falls through to the error behavior for ON EMPTY, and raises an error for ON ERROR.

// STRING: "true"
JSON_VALUE('{"a": true}', '$.a')

// BOOLEAN: true
JSON_VALUE('{"a": true}', '$.a' RETURNING BOOLEAN)

// STRING: "false"
JSON_VALUE('{"a": true}', 'lax $.b'
    DEFAULT FALSE ON EMPTY)

// STRING: "false"
JSON_VALUE('{"a": true}', 'strict $.b'
    DEFAULT FALSE ON ERROR)