Added set-prefix command.

commands/prefix.js

+const Discord = require('discord.js');
+exports.run = async(client, message, args, connection) => {
+  var sql = `SELECT prefix FROM prefixes WHERE guildid ='${message.guild.id}'`;
+  var sql2 = "";
+  var sql3 = "";
+  var sql4 = "";
+  let newprefix = args[0]
+  connection.query(sql, function (err, result) {
+    if (err) throw err;
+  if (result.length < 1) {
+       sql2 = `INSERT INTO prefixes (guildid, prefix) VALUES ('${message.guild.id}', '${newprefix}')`
+       connection.query(sql2, function (err, result) {
+         if (err) throw err;
+       });
+      message.channel.send('Setted prefix.')
+  } else {
+      sql3 = `DELETE FROM prefixes WHERE guildid ='${message.guild.id}'`
+      connection.query(sql3, function (err, result) {
+        if (err) throw err;
+      });
+      sql4 = `INSERT INTO prefixes (guildid, prefix) VALUES ('${message.guild.id}', '${newprefix}')`
+      connection.query(sql4, function (err, result) {
+        if (err) throw err;
+      });
+      message.channel.send('Setted new prefix.')
+  };
+});
+};
+
+module.exports.help = {
+    name: 'prefix',
+    aliases: ['set-prefix']
+};