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SQL union vs union all

· 阅读需 1 分钟
Deng Hongbing
Deng Hongbing
Full Stack Developer

x01​

Query:

select 1,3 union select 1,3

Result:

1, 3

x02​

Query:

select 1,3 union all select 1,3

Result:

1, 3
1, 3

总结​

union会合并去重(耗时)， union all不会去重。

Java8 Stream Example

· 阅读需 2 分钟
Deng Hongbing
Deng Hongbing
Full Stack Developer

Source​

Student xm = new Student();
xm.setId(11);
xm.setName("小明");
xm.setGender(1);

Student xh = new Student();
xh.setId(12);
xh.setName("小红");
xh.setGender(2);

Student xt = new Student();
xt.setId(13);
xt.setName("小天");
xt.setGender(1);

List<Student> list = new ArrayList<>();
list.add(xm);
list.add(xh);
list.add(xt);

x01​

Operate:

List<Integer> ids = list.stream().map(Student::getId).collect(Collectors.toList());

Result:

[11, 12, 13]

x02​

Operate:

Map<Integer, String> idNames = list.stream().collect(Collectors.toMap(Student::getId, Student::getName));

Result:

{ "11": "小明", "12": "小红", "13": "小天" }

x03​

Operate:

Map<Integer, Student> idMap = list.stream().collect(Collectors.toMap(Student::getId, Function.identity()));

Result:

{
  "11": { "id": 11, "name": "小明", "gender": 1 },
  "12": { "id": 12, "name": "小红", "gender": 2 },
  "13": { "id": 13, "name": "小天", "gender": 1 }
}

x04​

Operate:

Map<Integer, List<Student>> genderListMap = list.stream().collect(Collectors.groupingBy(Student::getGender));

Result:

{
  "1": [
    { "id": 11, "name": "小明", "gender": 1 },
    { "id": 13, "name": "小天", "gender": 1 }
  ],
  "2": [{ "id": 12, "name": "小红", "gender": 2 }]
}

x05​

Operate:

Map<Integer, List<String>> genderListNameMap = list.stream().collect(Collectors.groupingBy(Student::getId, Collectors.mapping(Student::getName, Collectors.toList())));

Result:

{
  "1": ["小明", "小天"],
  "2": ["小红"]
}

Let’s Encrypt Certbot Nginx renew

· 阅读需 1 分钟
Deng Hongbing
Deng Hongbing
Full Stack Developer

Let’s Encrypt是个免费证书，每次只有 90 天有效

环境​

CentOS7 Nginx

安装 certbot 和 nginx 插件​

yum install certbot python3-certbot-nginx

生成证书​

certbot --nginx

更新证书​

certbot renew --dry-run

自动更新​

crontab -e

0 3 * * * "certbot renew --dry-run && nginx -s reload"