Spark SQL----LIMIT子句

一、描述

LIMIT子句用于约束SELECT语句返回的行数。通常，此子句与ORDER BY一起使用，以确保结果是确定性的。

二、语法

LIMIT { ALL | integer_expression }

三、参数

• ALL
  如果指定，查询将返回所有行。换句话说，如果指定了此选项，则不应用任何限制。
• integer_expression
  指定返回整数的foldable表达式。

四、例子

CREATE TABLE person (name STRING, age INT);
INSERT INTO person VALUES
    ('Zen Hui', 25),
    ('Anil B', 18),
    ('Shone S', 16),
    ('Mike A', 25),
    ('John A', 18),
    ('Jack N', 16);

-- Select the first two rows.
SELECT name, age FROM person ORDER BY name LIMIT 2;
+------+---+
|  name|age|
+------+---+
|Anil B| 18|
|Jack N| 16|
+------+---+

-- Specifying ALL option on LIMIT returns all the rows.
SELECT name, age FROM person ORDER BY name LIMIT ALL;
+-------+---+
|   name|age|
+-------+---+
| Anil B| 18|
| Jack N| 16|
| John A| 18|
| Mike A| 25|
|Shone S| 16|
|Zen Hui| 25|
+-------+---+

-- A function expression as an input to LIMIT.
SELECT name, age FROM person ORDER BY name LIMIT length('SPARK');
+-------+---+
|   name|age|
+-------+---+
| Anil B| 18|
| Jack N| 16|
| John A| 18|
| Mike A| 25|
|Shone S| 16|
+-------+---+

-- A non-foldable expression as an input to LIMIT is not allowed.
SELECT name, age FROM person ORDER BY name LIMIT length(name);
org.apache.spark.sql.AnalysisException: The limit expression must evaluate to a constant value ...