我是 lex/yacc 的新手，我正在使用 lex/yacc 编写 SQL 解析器。但是对于特定的 JOIN 查询（如下所列），我的解析器故意访问“select_statement”规则而不是“nested_join_statement”规则。
我得到了这个查询的正确输出：SELECT * FROM sample1 JOIN sample2 ON sample1.C1 = sample2.C4;
（这将进入'join_statement rule'，满足其中的simple_join规则）
但是当我尝试这个查询时：SELECT * FROM (SELECT * FROM (SELECT * FROM sample1)T8)AS temp1 JOIN (SELECT * FROM (SELECT * FROM sample2)T9)as temp2 ON temp1.C1 = temp2.C4;
（理想情况下，这应该转到“nested_join_statement”，但它会转到“select statement”规则中的“SELECT selection FROM LPAREN select_statement2 RPAREN VAR”规则，我收到以下错误消息：错误：语法错误，意外 AS，期待 VAR）
我将 nested_join_statement 的优先级置于 select_statement 之上，但仍然出现此错误。我不明白为什么。
lex 中的 VAR 定义为 [A-Za-z][A-Za-z0-9_#-]*
任何帮助，将不胜感激。生无可恋。
manipulation_statement: NEWLINE
| join_statement SEMICOLON
| nested_join_statement SEMICOLON
| select_statement SEMICOLON { flag=0;q=0;}
| join_statement SEMICOLON NEWLINE
| nested_join_statement SEMICOLON NEWLINE
| select_statement SEMICOLON NEWLINE { flag=0;q=0;}
;
nested_join_statement: two_nest_select_join { for(x=0;x<q;x++) strcpy(sj[x], ""); i=0;j=0;vardot=0;q=0;flag=0;nest=0;}
;
join_statement:
simple_join { for(x=0;x<q;x++) strcpy(sj[x], ""); i=0;j=0;vardot=0;q=0;flag=0;}
| simple_join_nest_select { for(x=0;x<q;x++) strcpy(sj[x], ""); i=0;j=0;vardot=0;q=0;flag=0;}
;
simple_join: SELECT selection FROM VAR JOIN VAR ON VAR DOTS VAR EQUAL VAR DOTS VAR { printf("inside simple join\n");
if(strcmp($4,$8) == 0) join_table($4,$6,sj,q,$10,$14,"","");
else join_table($4,$6,sj,q,$14,$10,"",""); p=q;}
;
simple_join_nest_select: SELECT selection FROM VAR JOIN LPAREN select_statement RPAREN AS VAR ON VAR DOTS VAR EQUAL VAR DOTS VAR {
if (strcmp($4,$12) == 0) join_table($4,"_temp_",sj,q,$14,$18,"","");
else join_table($4,"_temp_",sj,q,$18,$14,"",""); p=q;}
;
two_nest_select_join: SELECT selection FROM LPAREN select_statement RPAREN AS VAR JOIN LPAREN select_statement RPAREN AS VAR ON VAR DOTS VAR EQUAL VAR DOTS VAR {
join_table("temp1","temp2",sj,q,$18,$22,"",""); }
;
select_statement: SELECT selection FROM VAR WHERE where_clause { select_table($4,s,i,whr_var,whr_val);
for(x=0;x<i;x++) strcpy(s[x],"");i=0; strcpy(whr_var,""); strcpy(whr_val,"");j=0;}
| SELECT selection FROM VAR { select_table($4,s,i,whr_var,whr_val);
for(x=0;x<i;x++) strcpy(s[x],"");i=0;j=0;}
| SELECT selection FROM LPAREN select_statement2 RPAREN VAR { printf("inner tab: %s\n", inner_tab); printf("dep:%d\n", dep+1);
for(x=0; x<i;x++) printf("%s\n", col_array[x]);
for(y=0; y<j;y++) printf("%d\n", col_count[y]);
strcpy(inner_tab,""); nest =1; dep=0;
for(x=0; x<i;x++) strcpy(col_array[x], "");
for(y=0; y<j;y++) col_count[x] =0;
i=0;j=0;k=0;m=0;}
;
select_statement2: SELECT selection2 FROM VAR { dep++; inner_tab=malloc(strlen($4)); strcpy(inner_tab,$4); }
| select_list
;
select_list: SELECT selection2 FROM LPAREN select_statement2 RPAREN VAR { dep++; }
;
selection2: ASTERISK { col_array[i] = $1; i++; m++; col_count[j] = m; j++; printf("in level:%d value of k:%d\n",j,m); k=0; m=0;}
| comma_list2 { col_count[j] = m; j++; printf("in level:%d value of k:%d\n",j,m); k=0; m=0; }
;
comma_list2: VAR { col_array[i] = $1; i++; m++;}
| comma_list2 COMMA VAR { col_array[i] = $3; i++; m++;
k=m;}
;
selection: ASTERISK { if(q != 0 || nest == 1) { for(x=0;x<j;x++) col_count[x]=0; i=0; j=0;flag=1;}
s[i] = $1; col_array[i] = $1; i++; col_count[j] = i; j++;
if(q == 0) {sj[q] = $1; q++;} printf("in level t:%d value of k:%d\n",j,i); printf("sj2:%s\n",sj[0]);}
| comma_list { if (flag ==1 ) {col_count[j] = i; j++; printf("in level 2:%d value of k:%d\n",j,i); k=0; m=0; printf("temp:");
for(x=0;x<i;x++) printf("%s ",s[x]); } else {col_count[j] = i; j++;} }
;
comma_list:
VAR { if(q != 0 || nest == 1) { for(x=0;x<j;x++) col_count[x]=0; i=0; j=0;flag=1;}
s[i] = $1; col_array[i] = $1; i++; if(q==0) {sj[q] = $1; q++; }}
| VAR DOTS VAR { strcpy(temp,""); strcat(temp,$1); strcat(temp,$2); strcat(temp,$3);
sj[q] = temp; q++;
printf("temps:"); for(x=0;x<q;x++) printf("%s\n",sj[x]);}
| comma_list COMMA vardot {if (flag == 1) { s[i] = $3; col_array[i] = $3, i++;} else {sj[q] = $3; q++; vardot++;s[i] = $3; col_array[i] = $3, i++;}
}
;