add exists syntax check.

src/main/java/com/alibaba/druid/sql/parser/SQLExprParser.java

@@ -1064,6 +1064,12 @@ public class SQLExprParser extends SQLParser {
             exprList(methodInvokeExpr.getParameters(), methodInvokeExpr);
         }
 
+        if (hash_lower == FnvHash.Constants.EXIST
+                && methodInvokeExpr.getParameters().size() == 1
+                && methodInvokeExpr.getParameters().get(0) instanceof SQLQueryExpr) {
+            throw new ParserException("exists syntax error.");
+        }
+
         if (lexer.token == Token.FROM) {
             lexer.nextToken();
             SQLExpr from = this.expr();

src/main/java/com/alibaba/druid/util/FnvHash.java

@@ -629,5 +629,6 @@ public final class FnvHash {
         long BUILD = fnv1a_64_lower("BUILD");
         long NOCACHE = fnv1a_64_lower("NOCACHE");
         long NOPARALLEL = fnv1a_64_lower("NOPARALLEL");
+        long EXIST = fnv1a_64_lower("EXIST");
     }
 }