Functional SQL: https://www.codewars.com/kata/545434090294935e7d0010ab

querier.js

/*
Solution of: https://www.codewars.com/kata/545434090294935e7d0010ab
*/

var query = function() {
	var self = {};
	
	var tables = [];
	var selector = null;

	var whereClauses = [];
	var havingClauses = [];
	
	var order = [];
	var group = [];
	
	var selectorAll = function(row) {
		return row;
	};
	
	self.select = function(e) {
		if (selector != null) throw new Error('Duplicate SELECT');
		selector = e || false; 
		return self; 
	};
	
	self.from = function() {
		if (tables.length > 0) throw new Error('Duplicate FROM');
		tables = Array.from(arguments);
		return self;
	};
	
	self.where = function() {
		whereClauses.push( Array.from(arguments) );
		return self;
	};
	
	self.having = function() {
		havingClauses.push( Array.from(arguments) );
		return self;
	};
	
	self.orderBy = function() {
		if (order.length > 0) throw new Error('Duplicate ORDERBY');
		order = Array.from(arguments);
		return self;
	};
	
	self.groupBy = function() {
		if (group.length > 0) throw new Error('Duplicate GROUPBY');
		group = Array.from(arguments);
		return self;
	};
	
	self.execute = function() {		
		var tmpdata = [];
		var gdata = [];

		var data = [];
		var t = 0;

		// JOIN

		if (tables.length > 1) {

			tables.forEach(function() {
        			data.push([ ]);
     			});

			tables[0].forEach(function(row, i) {
				for (t = 0; t < tables.length; t++) {
					data[t].push( tables[t][i] );
				}
			});

			tmpdata = [];
			(function traverseTable(D, t) {
				if (D.length === 0) {
					tmpdata.push( t.slice(0) );
				} else {
					for (var i = 0; i < D[0].length; i++) {
						t.push( D[0][i] );
						traverseTable( D.slice(1), t );
						t.splice(-1, 1);
					}
				}
			})(data, []);

			data = [];
			tmpdata.forEach(function(row, i) {
				if (whereClauses.every(function(orWhereClauses) {
					return orWhereClauses.some(function(whereClause) {
						return whereClause( row );
					});
				})) {
					data.push( row );
				}
			});

		} else if (tables.length === 1) {

			tables[0].forEach(function(row, i) {
				if (whereClauses.every(function(orWhereClauses) {
					return orWhereClauses.some(function(whereClause) {
						return whereClause( row );
					});
				})) {
					data.push( row );
				}
			});

		} else {
			data = [];
		}

		// Group

		if (group.length > 0) {

			var T = {};

			data.forEach(function(row) {
				var t = T;
				group.forEach(function(groupCallback) {
					var k = groupCallback(row);
					t[k] = t[k] || {}; t = t[k];
				});
				t._data = t._data || []; t._data.push(row);
			});

			(function traverse(node, R) {
				if (node._data != null) {
					node._data.forEach(function(e) { R.push(e); });
				} else {
					for (var k in node) {
						k = /\d+/.test(k) ? Number(k) : k;
						var row = [ k, [] ];
						traverse(node[k], row[1]);
						R.push(row);
					}
				}
			})(T, gdata);

			gdata.forEach(function(grow){ 
				if (havingClauses.every(function(orHavingClauses) {
					return orHavingClauses.some(function(havingClause) {
						return havingClause(grow);
					});
				})) {
					tmpdata.push(grow);
				}
			});
			data = tmpdata;

		}

		order.forEach(function(orderCallback) {
			data = data.sort(orderCallback);
		});
		
		return data.map( selector || selectorAll );
	};

	return self;
};