Homebrewでインストールしたmysqlがcommand not foundのときはシンボリックリンクを作成して解決

環境:Mac OS High Sierra 10.13.2

## 解決策
シンボリックリンクがないだけなので、手動で作ってあげればそれで解決です。

```
$ brew link mysql
```

が、少しハマった箇所もあるので、備忘録も兼ねて以下つらつらと書いちょります。


### 1. Homebrewでインストールしたmysqlが起動しない

ローカルでmysql使おうとするも、何やってもcommand not found・・・。
確かインストールはしてたはず・・・多分。

```
$ mysql
-bash: mysql: command not found
$ mysql -uroot
-bash: mysql: command not found
$ mysql.server start
-bash: mysql.server: command not found
```

### 2. `brew info mysql`で確認してみると・・・
やっぱり。インストールはしてるっぽい。

```
$ brew info mysql
mysql: stable 5.7.21 (bottled), devel 8.0.3-rc
Open source relational database management system
https://dev.mysql.com/doc/refman/5.7/en/
Conflicts with:
  mariadb (because mysql, mariadb, and percona install the same binaries.)
  mariadb-connector-c (because both install plugins)
  mysql-cluster (because mysql, mariadb, and percona install the same binaries.)
  mysql-connector-c (because both install MySQL client libraries)
  percona-server (because mysql, mariadb, and percona install the same binaries.)
/usr/local/Cellar/mysql/5.7.20 (324 files, 233.7MB)
  Poured from bottle on 2017-12-24 at 19:16:31
/usr/local/Cellar/mysql/5.7.21 (323 files, 233.9MB)
  Poured from bottle on 2018-01-21 at 15:29:58
From: https://github.com/Homebrew/homebrew-core/blob/master/Formula/mysql.rb
==> Dependencies
Build: cmake ✘
Required: openssl ✔
==> Requirements
Required: macOS >= 10.7 ✔
==> Options
--with-debug
	Build with debug support
--with-embedded
	Build the embedded server
--with-local-infile
	Build with local infile loading support
--with-memcached
	Build with InnoDB Memcached plugin
--with-test
	Build with unit tests
--devel
	Install development version 8.0.3-rc
==> Caveats
We've installed your MySQL database without a root password. To secure it run:
    mysql_secure_installation

MySQL is configured to only allow connections from localhost by default

To connect run:
    mysql -uroot

To have launchd start mysql now and restart at login:
  brew services start mysql
Or, if you don't want/need a background service you can just run:
  mysql.server start
```

### 3.`brew doctor`で確認
brewで困ったことがあったら、とりあえず`brew doctor`に聞いてみよう。すると・・・

```
$ brew doctor
Please note that these warnings are just used to help the Homebrew maintainers
with debugging if you file an issue. If everything you use Homebrew for is
working fine: please don't worry or file an issue; just ignore this. Thanks!

Warning: You have unlinked kegs in your Cellar
Leaving kegs unlinked can lead to build-trouble and cause brews that depend on
those kegs to fail to run properly once built. Run `brew link` on these:
  gdbm
  mysql
  pcre
```

mysqlへのシンボリックリンクがないようだ。

### 4.じゃあ、`brew link mysql`して解決っと・・・。あれ？

なんかエラーが起きてる。

```
$ brew link mysql
Linking /usr/local/Cellar/mysql/5.7.21...
Error: Could not symlink include/mysql
/usr/local/include is not writable.
```

`/usr/local/include is not writable.` 書き込めない？パーミッション？

```
$ sudo chmod g+w include
Password:
chmod: include: No such file or directory
```

そもそもディレクトリがなかったようだ。`mkdir`して同時にパーミッションも設定。

```
$ sudo mkdir /usr/local/include
$ sudo chmod g+w /usr/local/include
$ sudo chown :admin /usr/local/include
```

これで再び`brew link mysql`

```
$ brew link mysql
Linking /usr/local/Cellar/mysql/5.7.21... 93 symlinks created
```
よし、作れた！

### 解決！
これで`mysql`でエラーが出なければOK。

```
$ mysql
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 4
Server version: 5.7.20 Homebrew

Copyright (c) 2000, 2018, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql>
```
OK!

## おまけ

### Homebrewのシンボリックリンクの確認方法
`brew --prefix`で表示されるPATH配下の`bin`の中に入っています。

```
$ brew --prefix
/usr/local
$ ls /usr/local/bin
VBoxAutostart		git-credential-osxkeychain	mysql_embedded			perror
VBoxBalloonCtrl			git-cvsserver			mysql_install_db		pkg-config
VBoxDTrace			git-receive-pack		mysql_plugin			pyenv
VBoxHeadless			git-shell			mysql_secure_installation	pyenv-install
VBoxManage			git-subtree			mysql_ssl_rsa_setup		pyenv-uninstall
VBoxVRDP			git-upload-archive		mysql_tzinfo_to_sql		python-build
VirtualBox			git-upload-pack			mysql_upgrade			rbenv

以下略
```

mysqlもちゃんと入ってますね。