SQL是一种什么样的语言？

Question

SQL 是一种上下文无关语言还是某种其他类型的语言？

Answer 1

According to https://stackoverflow.com/a/31265136 SQL is not a regular language. The short explanation is that each select query looks like

 SELECT x FROM y WHERE z

and y can be another select query itself, so it cannot be simulated with finite-state machine. As mentioned before, there are some CFGs for SQL standarts in Backus–Naur Form , thereby SQL is nonregular context free language.

Answer 2

@aquinas wrote:

Do you mean is SQL also regular? CFG's encompass regular languages. So, they aren't mutually exclusive. To answer your question though, SQL is not a regular language.

@MSX wrote:

Just to clarify, a language is context-free when it is generated by a context-free grammar. There're SQL context-free grammar definitions online. Just google around and you'll find some. Here's one, for example.

Answer 3

Any CFG for SQL will do most of the work, but will always be slightly too permissive.

A good example of an SQL CFG comes from antlr: https://github.com/antlr/grammars-v4/blob/master/sql/plsql/PlSqlParser.g4#L1982

But at that line (1982), you see that in a values_clause , column values are recursively added regardless of how many columns might be specified, or how many values are in another row, which is invalid sql:

insert into xyz values
  (1, 'hello'),
  (2, 'bye'),
  (3, 'hi', 'uhm...'); -- invalid!

Try running it here: https://www.db-fiddle.com/f/6gwgcg6qBLKpFbCEPtB6dy/0

This syntax can never be fully encapsulated by a CFG, as it is equivalent to the { (a^n)(b^n)(c^n) : n ≥ 1 } language, which is famously not allowed in CFGs ( but allowed in CSGs )

You could argue this is a runtime error instead of a parsing error. But you can use the same argument for every language that is interpreted, so it's a bit of a grey area.

Answer 4

PL/SQL上下文中的相似可以是什么代码就是SQL或者PL/SQL因为如果我们把函数关键字定义为具体的，那么如果我们想控制可以使用哪些函数，这个上下文必须是已知的