为什么在访问函数返回的数组(在SELECT中)时需要添加括号? [英] why is it necessary to add parenthesis when accessing an array returned by a function (in SELECT)?
问题描述
来自(脚本)编程语言,我不明白为什么不起作用
Coming from (script) programming languages, I do not understand why this does not work
SELECT regexp_matches('hello world', '(he).*(wo)', 'g')[1];
,同时添加附加括号可以起作用(至少在Postgres中有效)。
while adding additional parenthesis does work (at least in Postgres).
SELECT (regexp_matches('hello world', '(he).*(wo)', 'g'))[1];
我会假设 regexp_matches $ c $返回的数组在这两种情况下都可以访问c>,但是我想我对SQL的语法缺乏了解。
I would have assumed that the array returned by regexp_matches
is accessed in both cases, but I guess my syntax understanding of SQL is lacking here.
那为什么我们需要括号?
推荐答案
括号中的内容非常高优先级,因此您的查询被解析为
the brackets have a pretty high precedence , so your query is parsed as
SELECT regexp_matches(('hello world', '(he).*(wo)', 'g')[1]);
这没有意义,您想在调用<$ c的结果上应用索引$ c> regexp_matches(),因此您必须添加括号以将regexp_matches调用分组。
This doesn't make sense, you want to apply the index on the result of calling regexp_matches()
, so you have to add parenthesis to group the regexp_matches call.
(请注意,这两个字符[]括在方括号中,您添加了括号,而不是括号
(Note these two characters [] are brackets, you added parenthesis, not brackets
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