如何为 select sql 语句编写语法？

Question

I am writing a grammar for an SQL parser and I've been stuck on this for a while now-

F:      FETCH  fields  FROM  tables  Conditions
;

fields:     ALL
|           ids
;

ids:        ID ids_
;

ids_:       ',' ID ids_
|            { /*empty*/ }
;

tables:     ID
;

Conditions:     WHERE  ConditionList 
|               { /*empty*/ }
;

ConditionList:      Condition ConditionList_
;

ConditionList_:     BoolOp Condition ConditionList_
|                   { /*empty*/ }
;

Condition:      Operand RELOP Operand
|               NOT Operand RELOP Operand
;

Operand:    ID
|           NUM
;

BoolOp:     AND
|           OR
;

For some reason when the lexer reads a FROM token, the parser terminates with an error. Here's the lex code-

FETCH{ printf("fetch ");            return FETCH;}
FROM        { printf("from ");  return UNIQUE;                                      }

ALL     { printf("all ");   return ALL;     }
WHERE       { printf("where ");     return WHERE;   }
AND         { printf("and ");   return AND;     }
OR      { printf("or ");    return OR;  }
NOT             { printf("not ");   return NOT; }
RelOp           { printf("%s", yytext);     yylval.string = strdup(yytext); return RELOP;   }
[0-9]*          {printf("num ");    return NUM; }
[_a-zA-Z][_a-zA-Z0-9]*      { printf("id ");        return ID;      }
{symbol}    { printf("%c ", yytext[0]); return yytext[0];       }
.                           {   }

RelOp is a pattern- RelOp ("<"|"<="|">"|">="|"=") and symbol is a pattern- symbol ("("|")"|",")

Answer 1

Your grammar starts with

F:      FETCH  fields  FROM  tables  Conditions

However, your lexer rules includes

FROM        { printf("from ");  return UNIQUE; }

Since UNIQUE is different from FROM , the grammar rule won't apply.

If those printf calls in your lexer are some kind of debugging attempt, they are not very useful since they won't tell you whether you are actually returning the correct token type (and value, in the cases where that is necessary). I strongly recommend using bison's trace feature to get an accurate view of what is going on. (Bison's trace will tell you which token type is being received by the parser, for example.)