diff --git a/package-lock.json b/package-lock.json
index 7ffcf1a..1f6f58a 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -1,12 +1,12 @@
 {
   "name": "spring-filter-query-builder",
-  "version": "0.1.0",
+  "version": "0.2.0",
   "lockfileVersion": 2,
   "requires": true,
   "packages": {
     "": {
       "name": "spring-filter-query-builder",
-      "version": "0.1.0",
+      "version": "0.2.0",
       "license": "MIT"
     }
   }
diff --git a/package.json b/package.json
index 3abc409..9fef5ea 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "spring-filter-query-builder",
-  "version": "0.1.0",
+  "version": "0.2.0",
   "description": "A simple Query Builder for Spring Filter",
   "main": "scr/index.js",
   "scripts": {
diff --git a/src/comparators.js b/src/comparators.js
index a43a576..53d6edc 100644
--- a/src/comparators.js
+++ b/src/comparators.js
@@ -10,8 +10,8 @@ class Comparator extends Item { //Abstract
 			throw `${new.target.name} parameter 'selector' must be a Function or a String!`;
 		}
 		if (value) {
-			if (!(typeof value === 'string' || value instanceof String)) {
-				throw `${new.target.name} parameter 'value' must be a String!`;
+			if (!(typeof value === 'string' || value instanceof String || typeof value === 'number' || value instanceof Number)) {
+				throw `${new.target.name} parameter 'value' must be a String or a Number!`;
 			}
 		}
 		this.selector = selector;
@@ -21,7 +21,11 @@ class Comparator extends Item { //Abstract
 
 	toString() {
 		if (this.value) {
-			return `${this.selector} ${this.comparatorKeyWord} ${this.value}`;
+			if (typeof this.value === 'number' || this.value instanceof Number) {
+				return `${this.selector} ${this.comparatorKeyWord} ${this.value}`;
+			} else {
+				return `${this.selector} ${this.comparatorKeyWord} '${this.value}'`;
+			}
 		} else {
 			return `${this.selector} ${this.comparatorKeyWord}`;
 		}
@@ -30,37 +34,37 @@ class Comparator extends Item { //Abstract
 
 class Like extends Comparator {
 	constructor(selector, value) {
-		super(selector, "~",value);
+		super(selector, "~", value);
 	}
 }
 
 class Equal extends Comparator {
 	constructor(selector, value) {
-		super(selector, ":",value);
+		super(selector, ":", value);
 	}
 }
 
 class NotEqual extends Comparator {
 	constructor(selector, value) {
-		super(selector, "!",value);
+		super(selector, "!", value);
 	}
 }
 
 class Gt extends Comparator {
 	constructor(selector, value) {
-		super(selector, ">",value);
+		super(selector, ">", value);
 	}
 }
 
 class Ge extends Comparator {
 	constructor(selector, value) {
-		super(selector, ">:",value);
+		super(selector, ">:", value);
 	}
 }
 
 class Lt extends Comparator {
 	constructor(selector, value) {
-		super(selector, "<",value);
+		super(selector, "<", value);
 	}
 }
 class Le extends Comparator {
@@ -97,6 +101,11 @@ class In extends Comparator {
 	constructor(selector, ...values) {
 		super(selector, "IN", "('" + values.join("', '") + "')");
 	}
+
+	toString() {
+		return `${this.selector} ${this.comparatorKeyWord} ${this.value}`;
+	}
+
 }
 
 export { Comparator, Like, Equal, NotEqual, Gt, Ge, Lt, Le, IsNull, IsNotNull, IsEmpty, IsNotEmpty, In };
\ No newline at end of file
diff --git a/src/functions.js b/src/functions.js
index 302db76..5e0ab91 100644
--- a/src/functions.js
+++ b/src/functions.js
@@ -2,7 +2,7 @@ import { Item } from './item.js';
 import { Comparator } from './comparators.js';
 
 class Function extends Item { //Abstract
-	constructor(selector, functionKeyWord) {
+	constructor(functionKeyWord, selector) {
 		if (new.target === Comparator) {
 			throw new TypeError("Cannot construct " + new.target.name + " instances directly");
 		}
@@ -15,91 +15,96 @@ class Function extends Item { //Abstract
 	}
 
 	toString() {
-		return this.functionKeyWord + " (" + this.selector + ")";
+		if (this.selector) {
+			return this.functionKeyWord + "(" + this.selector + ")";
+		} else {
+			return this.functionKeyWord + "()";
+
+		}
 	}
 }
 
 class Absolute extends Function {
 	constructor(selector) {
-		super(selector, "ABSOLUTE");
+		super("ABSOLUTE", selector);
 	}
 }
 
 class Average extends Function {
 	constructor(selector) {
-		super(selector, "AVERAGE");
+		super("AVERAGE", selector);
 	}
 }
 
 class Min extends Function {
 	constructor(selector) {
-		super(selector, "MIN");
+		super("MIN", selector);
 	}
 }
 
 class Max extends Function {
 	constructor(selector) {
-		super(selector, "MAX");
+		super("MAX", selector);
 	}
 }
 
 class Sum extends Function {
 	constructor(selector) {
-		super(selector, "SUM");
+		super("SUM", selector);
 	}
 }
 
 class CurrentDate extends Function {
 	constructor() {
-		super("", "CURRENTDATE");
+		super("CURRENTDATE");
 	}
 }
 
 class CurrentTime extends Function {
 	constructor() {
-		super("", "CURRENTTIME");
+		super("CURRENTTIME");
 	}
 }
 
 class CurrentTimestamp extends Function {
 	constructor() {
-		super("", "CURRENTTIMESSTAMP");
+		super("CURRENTTIMESSTAMP");
 	}
 }
 
 class Size extends Function {
 	constructor(selector) {
-		super(selector, "SIZE");
+		super("SIZE", selector);
 	}
 }
 
 class Length extends Function {
 	constructor(selector) {
-		super(selector, "LENGTH");
+		super("LENGTH", selector);
 	}
 }
 
 class Trim extends Function {
 	constructor(selector) {
-		super(selector, "TRIM");
+		super("TRIM", selector);
 	}
 }
 
 class Upper extends Function {
 	constructor(selector) {
-		super(selector, "UPPER");
+		super("UPPER", selector);
 	}
 }
 
 class Lower extends Function {
 	constructor(selector) {
-		super(selector, "LOWER");
+		super("LOWER", selector);
 	}
 }
 
 class Concat extends Function {
-	constructor(selector) {
-		super(selector, "CONCAT");
+	constructor(...selectors) {
+		super("CONCAT", "('" + selectors.join(", ") + "')");
 	}
 }