Presto实战

136

｜

第

8

章

SELECT cast('a' as integer);

Query failed: Cannot cast 'a' to INT

SELECT try_cast('a' as integer);

_col0

------

NULL

(1 row)

8.10

　

SELECT

语句基础

SELECT

语句非常重要，它允许你以表的格式从一个或多个表中返回数据，也可以仅仅返回

一行或一个值。

Presto

的

SELECT

查询更加复杂，因为它可以包含来自不同

catalog

和

schema

的表，而这些

是完全不同的数据源。你在

7.6

节中已经了解了这一点。

下面我们将深入细节，全面了解其能力。让我们从语法概述开始：

[ WITH with_query [, ...] ]

SELECT [ ALL | DISTINCT ] select_expr [, ...]

[ FROM from_item [, ...] ]

[ WHERE condition ]

[ GROUP BY [ ALL | DISTINCT ] grouping_element [, ...] ]

[ HAVING condition]

[ { UNION | INTERSECT | EXCEPT } [ ALL | DISTINCT ] select ]

[ ORDER BY expression [ ASC | DESC ] [, ...] ]

[ LIMIT [ count | ALL ] ]

select_expr ...