一个比较清晰的SQL编写风格

前言

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SQL (Structured Query Language) 是具备数据操纵和数据定义等多种功能的数据库语言，这种语言具备交互性特色，能为用户提供极大的便利，数据库管理系统应充分利用SQL语言提升计算机应用系统的工做质量与效率。

身边不少人工做中常常和SQL打交道, 但是每人的编写SQL风格都独树一帜。

恰好在githup上看到一个不错的编码风格, 在这里给你们推荐一下。

mattm/sql-style-guide

介绍

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使用小写的SQL

小写SQL比大写SQL易读，并且没必要一直按住shift键。

-- Good
select * from users

-- Bad
SELECT * FROM users

-- Bad
Select * From users
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单行查询vs多行查询

如下状况最好将SQL写在同一行中:

• 查询全部列(*)或者只查询1列或者两列
• 查询语句没有额外复杂性

-- Good
select * from users

-- Good
select id from users

-- Good
select id, email from users

-- Good
select count(*) from users
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这样作的缘由很简单，当全部内容都在一行时，仍然很容易阅读。但一旦你开始添加更多的列或更复杂的代码，若是是多行代码就更容易阅读:

-- Good
select
    id,
    email,
    created_at
from users

-- Good
select *
from users
where email = 'example@domain.com 复制代码

对于具备1或2列的查询，能够将这些列放在同一行上。对于3+列，将每一个列名放在它本身的行上，包括第一项:

-- Good
select id, email
from users
where email like '%@gmail.com'

-- Good
select user_id, count(*) as total_charges
from charges
group by user_id

-- Good
select
    id,
    email,
    created_at
from users

-- Bad
select id, email, created_at
from users

-- Bad
select id,
    email
from users
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向左对齐

-- Good
select id, email
from users
where email like '%@gmail.com'

-- Bad
select id, email
  from users
 where email like '%@gmail.com'
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使用单引号

一些SQL方言，如BigQuery支持使用双引号，可是对于大多数方言，双引号最终将引用列名。所以，单引号更可取:

-- Good
select *
from users
where email = 'example@domain.com'

-- Bad
select *
from users
where email = "example@domain.com"
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使用 != 而不用 <>

由于!=读起来像“不等于”，更接近咱们大声说出来的方式。

-- Good
select count(*) as paying_users_count
from users
where plan_name != 'free'

-- Bad
select count(*) as paying_users_count
from users
where plan_name <> 'free'
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逗号应该放在行尾

-- Good
select
    id,
    email
from users

-- Bad
select
    id
    , email
from users
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缩进条件

当只有一个where条件时，将它保留在与where相同的行上:

select email
from users
where id = 1234
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当有多个缩进时，将每一个缩进比where更深一层。将逻辑运算符放在前一个条件的末尾:

select id, email
from users
where 
    created_at >= '2019-03-01' and 
    vertical = 'work'
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避免括号内的空格

-- Good
select *
from users
where id in (1, 2)

-- Bad
select *
from users
where id in ( 1, 2 )
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将in值的长列表分红多个缩进行

-- Good
select *
from users
where email in (
    'user-1@example.com',
    'user-2@example.com',
    'user-3@example.com',
    'user-4@example.com'
)
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表名应该是名词的复数形式

-- Good
select * from users
select * from visit_logs

-- Bad
select * from user
select * from visitLog
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列名应该是xxx_xxx格式

-- Good select id, email, timestamp_trunc(created_at, month) as signup_month from users

-- Bad select id, email, timestamp_trunc(created_at, month) as SignupMonth from users

列名的约定

• 布尔字段的前缀应该是is_、has_或does_。例如，is_customer、has_unsubscribe等。
• 仅限日期的字段应该以_date做为后缀。例如,report_date。
• Date+time字段应该以_at做为后缀。例如，created_at、posted_at等。

列排序约定

首先放置主键，而后是外键，而后是全部其余列。若是表中有任何系统列(created_at、updated_at、is_deleted等)，那么将它们放在最后。

-- Good
select
    id,
    name,
    created_at
from users

-- Bad
select
    created_at,
    name,
    id,
from users
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inner join 处理

最好显式，以便链接类型很是清楚:

-- Good
select
    email,
    sum(amount) as total_revenue
from users
inner join charges on users.id = charges.user_id

-- Bad
select
    email,
    sum(amount) as total_revenue
from users
join charges on users.id = charges.user_id
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对于join条件，将首先引用的表放在on以后

-- Good
select
    ...
from users
left join charges on users.id = charges.user_id
-- primary_key = foreign_key --> one-to-many --> fanout
  
select
    ...
from charges
left join users on charges.user_id = users.id
-- foreign_key = primary_key --> many-to-one --> no fanout

-- Bad
select
    ...
from users
left join charges on charges.user_id = users.id
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单个链接条件应该与链接位于同一行

-- Good
select
    email,
    sum(amount) as total_revenue
from users
inner join charges on users.id = charges.user_id
group by email

-- Bad
select
    email,
    sum(amount) as total_revenue
from users
inner join charges
on users.id = charges.user_id
group by email
复制代码

当你有多个链接条件时，把它们放在各自的缩进行:

-- Good
select
    email,
    sum(amount) as total_revenue
from users
inner join charges on 
    users.id = charges.user_id and
    refunded = false
group by email
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除非必要，不然不要包含表名

-- Good
select
    id,
    name
from companies

-- Bad
select
    companies.id,
    companies.name
from companies
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始终重命名聚合和函数包装的参数

-- Good
select count(*) as total_users
from users

-- Bad
select count(*)
from users

-- Good
select timestamp_millis(property_beacon_interest) as expressed_interest_at
from hubspot.contact
where property_beacon_interest is not null

-- Bad
select timestamp_millis(property_beacon_interest)
from hubspot.contact
where property_beacon_interest is not null
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在布尔条件需显示写出

-- Good
select * from customers where is_cancelled = true
select * from customers where is_cancelled = false

-- Bad
select * from customers where is_cancelled
select * from customers where not is_cancelled
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用as 重命名列名

-- Good
select
    id,
    email,
    timestamp_trunc(created_at, month) as signup_month
from users

-- Bad
select
    id,
    email,
    timestamp_trunc(created_at, month) signup_month
from users
复制代码

group by 时写出列名而不要用序号

-- Good
select user_id, count(*) as total_charges
from charges
group by user_id

-- Bad
select
    user_id,
    count(*) as total_charges
from charges
group by 1
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分组的列首先显示出来

-- Good
select
  timestamp_trunc(com_created_at, year) as signup_year,
  count(*) as total_companies
from companies
group by signup_year

-- Bad
select
  count(*) as total_companies,
  timestamp_trunc(com_created_at, year) as signup_year
from companies
group by signup_year
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调整对齐case-when语句

每一个when应该在它本身的行上(case行上没有任何内容)，而且应该比case行缩进更深一层。then能够在同一条线上，也能够在它下面的本身的线上。

-- Good
select
    case
        when event_name = 'viewed_homepage' then 'Homepage'
        when event_name = 'viewed_editor' then 'Editor'
    end as page_name
from events

-- Good too
select
    case
        when event_name = 'viewed_homepage'
            then 'Homepage'
        when event_name = 'viewed_editor'
            then 'Editor'
    end as page_name
from events

-- Bad 
select
    case when event_name = 'viewed_homepage' then 'Homepage'
        when event_name = 'viewed_editor' then 'Editor'
    end as page_name
from events
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尽可能使用CTEs, 避免使用子查询

-- Good
with ordered_details as (

    select
        user_id,
        name,
        row_number() over (partition by user_id order by date_updated desc) as details_rank
    from billingdaddy.billing_stored_details

),

final as (

    select user_id, name
    from ordered_details
    where details_rank = 1

)

select * from final

-- Bad
select user_id, name
from (
    select
        user_id,
        name,
        row_number() over (partition by user_id order by date_updated desc) as details_rank
    from billingdaddy.billing_stored_details
) ranked
where details_rank = 1
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使用有意义的CTE名称

-- Good
with ordered_details as (

-- Bad
with d1 as (
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使用over()函数

你能够把它放在同一行上，也能够根据它的长度把它分红多行:

-- Good
select
    user_id,
    name,
    row_number() over (partition by user_id order by date_updated desc) as details_rank
from billingdaddy.billing_stored_details

-- Good
select
    user_id,
    name,
    row_number() over (
        partition by user_id
        order by date_updated desc
    ) as details_rank
from billingdaddy.billing_stored_details
复制代码

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