function query(){
	var wheres = []
  var select = -1
  var from = -1
  var orderBy = -1
  var groupBy = -1
  var havings = []
  function selectfn(fn){
    if(select == -1){
      select = fn
    }else{
      throw new Error('Duplicate SELECT')
    }
  }
  function fromfn(xs,ys){
    if(from == -1){
      if(ys == null){
        from = xs.concat([])
      }else{
        var r = []
        for(var i=0;i<xs.length;i++){
          for(var j=0;j<ys.length;j++){
            r.push([xs[i],ys[j]])
          }
        }
        from = r
      }
    }else{
      throw new Error('Duplicate FROM')
    }
  }
  function wherefn(){
    var argfn = [].slice.apply(arguments)
    wheres.push(function(a){
      return anyPass(argfn,a)
    })
  }
  function orderByfn(fn){
   if(orderBy == -1){
     orderBy = fn
   }else{
     throw new Error('Duplicate ORDERBY')
   }
  }
  function havingfn(fn){
    var argfn = [].slice.apply(arguments)
    havings.push(function(a){
      return anyPass(argfn,a)
    })
  }
  function anyPass(fns,a){
    if(fns.length == 0) return false
    else return fns[0](a) ? true : anyPass(fns.slice(1),a)
  }
  function allPass(fns,a){
    if(fns.length==0) return true
    else return fns[0](a) ? allPass(fns.slice(1),a) : false
  }
  function groupByfn(fn){
    if(groupBy == -1){
      groupBy = [].slice.apply(arguments)
    }else{
      throw new Error('Duplicate GROUPBY')
    }
  }
  function groupByItem(gys,xs){
    if(gys.length == 0) return xs
    else void null
    var r = []
    var groupBy = gys[0]
    xs.forEach(a=>{
      var ritem = null
      r.forEach(ra=>{
        if(ra[0] == groupBy(a)){
          ritem = ra
        }else{
          void null
        }
      })
      if(ritem){
        ritem[1].push(a)
      }else{
        r.push([groupBy(a),[a]])
      }
    })
    return r.map(a=>[a[0],...[groupByItem(gys.slice(1),a[1])]])
  }
  function execute(){
    from = from == -1 ? [] : from
    if(wheres.length != 0){
      from = from.filter(function(a){
        return allPass(wheres,a)
      })
    }
    if(groupBy != -1&&groupBy != null){
      
      from = groupByItem(groupBy,from)
    }
    if(havings.length != 0){
      from = from.filter(function(a){
        return allPass(havings,a)
      })
    }
    if(select != -1&&select != null){
      from = from.map(select)
    }
    if(orderBy != -1&&orderBy != null){
      from = from.sort(orderBy)
    }
    
    return from
  }
  function bindthis(fn){
    return function(){
      fn.apply(null,[].slice.apply(arguments))
      return this
    }
  }
  return {
    select:bindthis(selectfn),
    from:bindthis(fromfn),
    where:bindthis(wherefn),
    orderBy:bindthis(orderByfn),
    groupBy:bindthis(groupByfn),
    having:bindthis(havingfn),
    execute:execute
  }
}

Test.describe("SQL tests", function() {
    Test.it("Basic SELECT tests", function() {
      var numbers = [1, 2, 3];
      Test.assertSimilar(query().select().from(numbers).execute(), numbers);
      Test.assertSimilar(query().select().execute(), [], 'No FROM clause produces empty array');
      Test.assertSimilar(query().from(numbers).execute(), numbers, 'SELECT can be omited');
      Test.assertSimilar(query().execute(), []);
      Test.assertSimilar(query().from(numbers).select().execute(), numbers, 'The order does not matter');
    });
    
    Test.it("Basic SELECT and WHERE over objects", function() {
      var persons = [
        {name: 'Peter', profession: 'teacher', age: 20, maritalStatus: 'married'},
        {name: 'Michael', profession: 'teacher', age: 50, maritalStatus: 'single'},
        {name: 'Peter', profession: 'teacher', age: 20, maritalStatus: 'married'},
        {name: 'Anna', profession: 'scientific', age: 20, maritalStatus: 'married'},
        {name: 'Rose', profession: 'scientific', age: 50, maritalStatus: 'married'},
        {name: 'Anna', profession: 'scientific', age: 20, maritalStatus: 'single'},
        {name: 'Anna', profession: 'politician', age: 50, maritalStatus: 'married'}
      ];
      
      Test.assertSimilar(query().select().from(persons).execute(), persons);
      
      function profession(person) {
        return person.profession;
      }

      //SELECT profession FROM persons
      Test.assertSimilar(query().select(profession).from(persons).execute(),  ["teacher","teacher","teacher","scientific","scientific","scientific","politician"]);
      Test.assertSimilar(query().select(profession).execute(), [], 'No FROM clause produces empty array');
      
      
      function isTeacher(person) {
        return person.profession === 'teacher';
      }
      
      //SELECT profession FROM persons WHERE profession="teacher" 
      Test.assertSimilar(query().select(profession).from(persons).where(isTeacher).execute(), ["teacher", "teacher", "teacher"]);
      
            
      //SELECT * FROM persons WHERE profession="teacher" 
      Test.assertSimilar(query().from(persons).where(isTeacher).execute(), persons.slice(0, 3));
      
      function name(person) {
        return person.name;
      }
      
      //SELECT name FROM persons WHERE profession="teacher" 
      Test.assertSimilar(query().select(name).from(persons).where(isTeacher).execute(), ["Peter", "Michael", "Peter"]); 
      Test.assertSimilar(query().where(isTeacher).from(persons).select(name).execute(), ["Peter", "Michael", "Peter"]); 
            
    });
    
        
    Test.it('GROUP BY tests', function() {

      var persons = [
        {name: 'Peter', profession: 'teacher', age: 20, maritalStatus: 'married'},
        {name: 'Michael', profession: 'teacher', age: 50, maritalStatus: 'single'},
        {name: 'Peter', profession: 'teacher', age: 20, maritalStatus: 'married'},
        {name: 'Anna', profession: 'scientific', age: 20, maritalStatus: 'married'},
        {name: 'Rose', profession: 'scientific', age: 50, maritalStatus: 'married'},
        {name: 'Anna', profession: 'scientific', age: 20, maritalStatus: 'single'},
        {name: 'Anna', profession: 'politician', age: 50, maritalStatus: 'married'}
      ];

      function profession(person) {
        return person.profession;
      }
    
      //SELECT * FROM persons GROUPBY profession <- Bad in SQL but possible in JavaScript
      Test.assertSimilar(query().select().from(persons).groupBy(profession).execute(), [["teacher",[{"name":"Peter","profession":"teacher","age":20,"maritalStatus":"married"},{"name":"Michael","profession":"teacher","age":50,"maritalStatus":"single"},{"name":"Peter","profession":"teacher","age":20,"maritalStatus":"married"}]],["scientific",[{"name":"Anna","profession":"scientific","age":20,"maritalStatus":"married"},{"name":"Rose","profession":"scientific","age":50,"maritalStatus":"married"},{"name":"Anna","profession":"scientific","age":20,"maritalStatus":"single"}]],["politician",[{"name":"Anna","profession":"politician","age":50,"maritalStatus":"married"}]]]); 
      
      function isTeacher(person) {
        return person.profession === 'teacher';
      }
      
      //SELECT * FROM persons WHERE profession='teacher' GROUPBY profession
      Test.assertSimilar(query().select().from(persons).where(isTeacher).groupBy(profession).execute(),  [["teacher",[{"name":"Peter","profession":"teacher","age":20,"maritalStatus":"married"},{"name":"Michael","profession":"teacher","age":50,"maritalStatus":"single"},{"name":"Peter","profession":"teacher","age":20,"maritalStatus":"married"}]]]); 

      function professionGroup(group) {
        return group[0];
      }
      
      //SELECT profession FROM persons GROUPBY profession
      Test.assertSimilar(query().select(professionGroup).from(persons).groupBy(profession).execute(), ["teacher","scientific","politician"]);

      function name(person) {
        return person.name;
      }
      
      //SELECT * FROM persons WHERE profession='teacher' GROUPBY profession, name
      Test.assertSimilar(query().select().from(persons).groupBy(profession, name).execute(),  [["teacher",[["Peter",[{"name":"Peter","profession":"teacher","age":20,"maritalStatus":"married"},{"name":"Peter","profession":"teacher","age":20,"maritalStatus":"married"}]],["Michael",[{"name":"Michael","profession":"teacher","age":50,"maritalStatus":"single"}]]]],["scientific",[["Anna",[{"name":"Anna","profession":"scientific","age":20,"maritalStatus":"married"},{"name":"Anna","profession":"scientific","age":20,"maritalStatus":"single"}]],["Rose",[{"name":"Rose","profession":"scientific","age":50,"maritalStatus":"married"}]]]],["politician",[["Anna",[{"name":"Anna","profession":"politician","age":50,"maritalStatus":"married"}]]]]]
);

      function age(person) {
        return person.age;
      }
      
      function maritalStatus(person) {
        return person.maritalStatus;
      }
      
      //SELECT * FROM persons WHERE profession='teacher' GROUPBY profession, name, age
      Test.assertSimilar(query().select().from(persons).groupBy(profession, name, age, maritalStatus).execute(), [["teacher",[["Peter",[[20,[["married",[{"name":"Peter","profession":"teacher","age":20,"maritalStatus":"married"},{"name":"Peter","profession":"teacher","age":20,"maritalStatus":"married"}]]]]]],["Michael",[[50,[["single",[{"name":"Michael","profession":"teacher","age":50,"maritalStatus":"single"}]]]]]]]],["scientific",[["Anna",[[20,[["married",[{"name":"Anna","profession":"scientific","age":20,"maritalStatus":"married"}]],["single",[{"name":"Anna","profession":"scientific","age":20,"maritalStatus":"single"}]]]]]],["Rose",[[50,[["married",[{"name":"Rose","profession":"scientific","age":50,"maritalStatus":"married"}]]]]]]]],["politician",[["Anna",[[50,[["married",[{"name":"Anna","profession":"politician","age":50,"maritalStatus":"married"}]]]]]]]]]);
      
      function professionCount(group) {
        return [group[0], group[1].length];
      }
      
      //SELECT profession, count(profession) FROM persons GROUPBY profession
      Test.assertSimilar(query().select(professionCount).from(persons).groupBy(profession).execute(),  [["teacher",3],["scientific",3],["politician",1]]);

      function naturalCompare(value1, value2) {
        if (value1 < value2) {
          return -1;
        } else if (value1 > value2) {
          return 1;
        } else {
          return 0;
        }
      }
      
      //SELECT profession, count(profession) FROM persons GROUPBY profession ORDER BY profession
      Test.assertSimilar(query().select(professionCount).from(persons).groupBy(profession).orderBy(naturalCompare).execute(), [["politician",1],["scientific",3],["teacher",3]]);
    });
    
    Test.it('Numbers tests', function() {

      function isEven(number) {
        return number % 2 === 0;
      }
      
      function parity(number) {
        return isEven(number) ? 'even' : 'odd';
      }
      
      function isPrime(number) {
        if (number < 2) {
          return false;
        }
        var divisor = 2;
        for(; number % divisor !== 0; divisor++);
        return divisor === number;
      }
      
      function prime(number) {
        return isPrime(number) ? 'prime' : 'divisible';
      }
      
      var numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9];    
      
      //SELECT * FROM numbers
      Test.assertSimilar(query().select().from(numbers).execute(), numbers);

      //SELECT * FROM numbers GROUP BY parity
      Test.assertSimilar(query().select().from(numbers).groupBy(parity).execute(), [["odd",[1,3,5,7,9]],["even",[2,4,6,8]]]);

      //SELECT * FROM numbers GROUP BY parity, isPrime
      Test.assertSimilar(query().select().from(numbers).groupBy(parity, prime).execute(), [["odd",[["divisible",[1,9]],["prime",[3,5,7]]]],["even",[["prime",[2]],["divisible",[4,6,8]]]]]);

      function odd(group) {
        return group[0] === 'odd';
      }
      
      //SELECT * FROM numbers GROUP BY parity HAVING 
      Test.assertSimilar(query().select().from(numbers).groupBy(parity).having(odd).execute(), [["odd",[1,3,5,7,9]]]);      
      
      function descendentCompare(number1, number2) {
        return number2 - number1;
      }
      
      //SELECT * FROM numbers ORDER BY value DESC 
     Test.assertSimilar(query().select().from(numbers).orderBy(descendentCompare).execute(), [9,8,7,6,5,4,3,2,1]);

      function lessThan3(number) {
        return number < 3;
      }
      
      function greaterThan4(number) {
        return number > 4;
      }
      
      //SELECT * FROM number WHERE number < 3 OR number > 4
      Test.assertSimilar(query().select().from(numbers).where(lessThan3, greaterThan4).execute(),  [1,2,5,6,7,8,9]);
    });
    
    Test.it('Frequency tests', function() {
      
      var persons = [
        ['Peter', 3],
        ['Anna', 4],
        ['Peter', 7],
        ['Michael', 10]
      ];
      
      function nameGrouping(person) {
        return person[0];
      }
      
      function sumValues(value) {
        return [value[0], value[1].reduce(function(result, person) {
          return result + person[1];
        }, 0)];
      }

      function naturalCompare(value1, value2) {
        if (value1 < value2) {
          return -1;
        } else if (value1 > value2) {
          return 1;
        } else {
          return 0;
        }
      }      
      //SELECT name, sum(value) FROM persons ORDER BY naturalCompare GROUP BY nameGrouping
      Test.assertSimilar(query().select(sumValues).from(persons).orderBy(naturalCompare).groupBy(nameGrouping).execute(),  [["Anna",4],["Michael",10],["Peter",10]]);
      
      var numbers = [1, 2, 1, 3, 5, 6, 1, 2, 5, 6];
      
      function id(value) {
        return value;
      }
      
      function frequency(group) {
        return { value: group[0], frequency: group[1].length };      
      }
      
      //SELECT number, count(number) FROM numbers GROUP BY number
      Test.assertSimilar(query().select(frequency).from(numbers).groupBy(id).execute(), [{"value":1,"frequency":3},{"value":2,"frequency":2},{"value":3,"frequency":1},{"value":5,"frequency":2},{"value":6,"frequency":2}]);
      
      function greatThan1(group) {
        return group[1].length > 1;
      }
      
      function isPair(group) {
        return group[0] % 2 === 0;
      }
      
      //SELECT number, count(number) FROM numbers GROUP BY number HAVING count(number) > 1 AND isPair(number)
      Test.assertSimilar(query().select(frequency).from(numbers).groupBy(id).having(greatThan1).having(isPair).execute(),  [{"value":2,"frequency":2},{"value":6,"frequency":2}]);
      
      
    });
    
    Test.it('join tests', function() {
      var teachers = [
        {
          teacherId: '1',
          teacherName: 'Peter'
        },
        {
          teacherId: '2',
          teacherName: 'Anna'
        }
      ];
      
      
      var students = [
        {
          studentName: 'Michael',
          tutor: '1'
        },
        {
          studentName: 'Rose',
          tutor: '2'
        }
      ];
      
      function teacherJoin(join) {
        return join[0].teacherId === join[1].tutor;
      }
      
      function student(join) {
        return {studentName: join[1].studentName, teacherName: join[0].teacherName};
      }
      
      //SELECT studentName, teacherName FROM teachers, students WHERE teachers.teacherId = students.tutor
      Test.assertSimilar(query().select(student).from(teachers, students).where(teacherJoin).execute(), [{"studentName":"Michael","teacherName":"Peter"},{"studentName":"Rose","teacherName":"Anna"}]);
      
      
      var numbers1 = [1, 2];
      var numbers2 = [4, 5];

      Test.assertSimilar(query().select().from(numbers1, numbers2).execute(), [[1,4],[1,5],[2,4],[2,5]]);
      
      function tutor1(join) {
        return join[1].tutor === "1";
      }
      
      //SELECT studentName, teacherName FROM teachers, students WHERE teachers.teacherId = students.tutor AND tutor = 1 
      Test.assertSimilar(query().select(student).from(teachers, students).where(teacherJoin).where(tutor1).execute(), [{"studentName":"Michael","teacherName":"Peter"}]);

    });
    
    Test.it('Duplication exception tests', function() {
      function checkError(fn, duplicate) {
        try {
          fn();
          Test.expect(false, 'An error should be throw');
        } catch (e) {
          Test.expect(e instanceof Error);
          Test.assertEquals(e.message, 'Duplicate ' + duplicate);
        }
      }
      
        
      function id(value) {
        return value;
      }    
      
      checkError(function() { query().select().select().execute(); }, 'SELECT');
      checkError(function() { query().select().from([]).select().execute(); }, 'SELECT');
      checkError(function() { query().select().from([]).from([]).execute(); }, 'FROM');
      checkError(function() { query().select().from([]).orderBy(id).orderBy(id).execute(); }, 'ORDERBY');
      checkError(function() { query().select().groupBy(id).from([]).groupBy(id).execute(); }, 'GROUPBY');
      
    });
});