javascript - IF EXISTS clause Local SQL -

the code below works fine. however, know how know how ensure 'late' entry added lates table if student name exists in student table. tried several versions of 'if exists' , 'where exists' can't seem syntax correct. return javascript error if name not exist in student table. desired change in addlate function.

    <fieldset>         <legend><b>details</b></legend>         <label>first name </label><input id = "firstname" type="text" autofocus="" placeholder="enter first name"><br><br>         <label>last name </label><input id = "lastname" type="text" autofocus="" placeholder="enter last name"><br><br>     </fieldset>      <fieldset>         <legend><b>information</b></legend>         <label> current date:</label>         <input type="text" id="datelate"/><br><br>         <label> detention date:</label>         <input id = "detentiondate" type="date" ><br><br>         <label>time</label>             <select id="mora">                 <option value="am">morning</option>                 <option value="pm">afternoon</option>             </select>         <br> <br>         <label> reason:</label>         <textarea id = "reason" rows="2" cols="60"></textarea><br><br>     </fieldset>      <br>     <input type="reset" value="reset">      <button type="button" id="addlate" onclick="addlate();">add late</button>      </body>  <script>  if (window.opendatabase) {     var mydb = opendatabase("students2_db", "0.1", "a database of students", 1024 * 1024);      mydb.transaction(function (t) {          t.executesql("create table if not exists student (id integer primary key asc, fname text, lname text, mclass text, aclass text, com text, lates integer default 0)");         t.executesql("create table if not exists lates (lid integer primary key asc, flname text, llname text, time text, reason text, date text, nextdet text)");     });  } else {     alert("websql not supported browser!"); }  function clearfunction() {     document.getelementbyid("mora").value = ""; }  function datefunction() {     var today = new date();     var dd = today.getdate();     var mm = today.getmonth()+1; //january 0!     var yyyy = today.getfullyear();      if(dd<10) {         dd='0'+dd     }       if(mm<10) {         mm='0'+mm     }       today = dd+'-'+mm+'-'+yyyy;     document.getelementbyid('datelate').value= today; }  function addlate() {      if (mydb) {          var flname = document.getelementbyid("firstname").value;         var llname = document.getelementbyid("lastname").value;         var date = document.getelementbyid("datelate").value;         var nextdet = document.getelementbyid("detentiondate").value;         var time = document.getelementbyid("mora").value;         var reason = document.getelementbyid("reason").value;          if (flname !== "" && date !== "" && nextdet !== "" && llname !== "" && reason !== "" && time !== "") {              mydb.transaction(function (t) {                 t.executesql("insert lates (flname,llname,time,reason,date,nextdet) values (?,?,?,?,?,?)" , [flname,llname,time,reason,date,nextdet]);                 t.executesql("update student set lates = lates + 1 lname =? ", [llname]);                 alert("entry succesfully added");                 document.getelementbyid("firstname").value = "";                 document.getelementbyid("lastname").value = "";                 document.getelementbyid("detentiondate").value = "";                 document.getelementbyid("reason").value = "";             });         } else {             alert("you must fill out empty information!");         }     } else {         alert("db not found, browser not support web sql!");     } }   </script> 

you can use simple select query using student id check if student within student database, use count of rows returned determine if student exists. if rows returned > 0, student exists, else, student not exist.

something this:

select id student id = 'student's id'