FROM not rejected in column position

[aidanhs]

Parsing SELECT FROM abc succeeds with:

AST: [                                                                         
    Query(                                                                     
        Query {                                                                
            ctes: [],                                                          
            body: Select(                                                      
                Select {                                                       
                    distinct: false,                                           
                    projection: [                                              
                        ExprWithAlias {                                        
                            expr: Identifier(                                  
                                "FROM",                                        
                            ),                                                 
                            alias: "issues",                                   
                        },                                                     
                    ],                                                         
                    from: [],                                                  
                    selection: None,                                           
                    group_by: [],                                              
                    having: None,                                              
                },                                                             
            ),                                                                 
            order_by: [],                                                                                                                                     
            limit: None,                                                       
            offset: None,                                                      
            fetch: None,                                                       
        },                                                                     
    ),                                                                         
]

It seems suprising that parsing accepted FROM as a column identifier! The SQL engines I've quickly tried out (sqlite locally, a handful of online validators) all seem to indicate that it's invalid - maybe it should be rejected?

[nickolay]

I agree that accepting SELECT FROM abc is surprising and I agree that this is probably invalid in many (if not all) dialects, so I would consider a fix if someone were to propose one.

But it's not obvious to me how this should be changed or why.

---

Some context:

There have been precedents for adding validations to reject such obviously incorrect code, e.g. in https://github.com/andygrove/sqlparser-rs/pull/118, but also for disabling such validations because some dialect was found to accept what we thought was universally rejected (https://github.com/andygrove/sqlparser-rs/issues/154).

In general, sqlparser does not promise to be a validating parser, that definitely rejects "invalid" SQL, because there's no commonly accepted definition of such a thing. I did some research on "reserved keywords" in various dialects before I implemented the current behavior:

• the SQL specification, for example, reserves more keywords than any other real-world engine (e.g. position or period)
• what it means for a keyword to be "reserved" also depends on the dialect. In Postgres, for example, a keyword can be fully reserved, allowed only as a function or type name, allowed only as a column or table name, or both. And even reserved keywords can appear after AS, e.g. SELECT 1 AS FROM.

[aidanhs]

Your reasoning is pretty convincing - thanks for laying it out so clearly!