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Simplify case statement formatting guidelines.
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@sean-rose
sean-rose committed Nov 30, 2021
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- [General guidelines](#general-guidelines)
- [Syntax](#syntax)
- [Joins](#joins)
- [Case statements](#case-statements)
- [CTEs](#ctes)
- [Naming](#naming)
- [Formatting](#formatting)
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<br>

### Case statements

- `then` can be at the end of single-line `when` clauses or go on its own line indented from `when`.
- `else` should be indented the same amount as the when clauses.
- `end` is aligned with case.
- In multi-line `when` clauses all subsequent lines should be indented.

<br>

#### Keep case statements with one short condition on a single line

```sql
/* Good */
case when customers.status = 1 then 'Active' else 'Inactive' end as column_name

/* Bad */
case
when customers.status = 1 then 'Active' else 'Inactive'
end as column_name
```

<br>

#### Use separate lines for long conditions

Lines should ideally not be longer than 120 characters. If a long condition results in more than 120 characters:
- Put each `when` clause on a new line, indented from `case`.
- `then` clauses can flow to a new line, indented from the `when` clause.
- Put the `else` clause on the same level as the `when` clauses.

```sql
/* Good */
case
when [very very very very very very very long condition]
then [result]
else [result]
end as column_name

/* Bad */
case when [very very very very very very very long condition] then [result] else [result] end as column_name

/* Bad */
case when [very very very very very very very long condition]
then [result]
else [result] end as column_name

```

<br>

#### Use seperate lines when there are multiple conditions

- Put the conditions on their own lines, indented from `case`.
- Put the `else` clause on the same level as the conditions.

```sql
/* Good*/
case
when orders.status = 1 then 'Completed'
when orders.status = 2 then 'Processing'
when orders.status = 3 then 'Cancelled'
else 'Failed'
end as column_name

/* Good */
case orders.status
when 1 then 'Completed'
when 2 then 'Processing'
when 3 then 'Cancelled'
else 'Failed'
end as column_name

/* Bad */
case orders.status when 1 then 'Completed' when 2 then 'Processing' when 3 then 'Cancelled' else 'Failed' end as column_name

```

<br>

#### Use separate lines for multiple conditions in a when clause

- Put each `when` clause on a new line, indented from `case`.
- Put the first condition on the same line as `when`. Put other conditions in a new line, indented from `when`.
- Boolean expressions (`and`, `or` etc.) should be after, not before newlines.
- Put the `else` clause on the same level as the `when` conditions.

```sql

/* Good */
case
when
orders.refund = 1
and orders.total_amount = orders.refund_amount
then 'Complete refund'
else 'Partial refund'
end as column_name

/* Bad */
case
when
orders.refund = 1 and
orders.total_amount = orders.refund_amount
then 'Complete refund'
else 'Partial refund'
end as column_name
```

<br>

#### Use separate lines for conditions with long list matching

For long lists:
- Put each `when` clause on a new line, indented from `case`.
- List values should be on their own lines, indented from the enclosing `when` condition.
- The closing parenthesis and the `then` clause should be aligned with the list.

```sql

/* Good */
case
when
plan_name in (
'monthly'
, 'bimonthly'
, 'quarterly'
, 'yearly'
)
then 'Paying customer'
else 'Guest customer'
end as column_name

/* Bad */
case
when plan_name in ('monthly', 'bimonthly', 'quarterly', 'yearly') then 'Paying customer' else 'Guest customer'
end as column_name
```

<br>

#### Use separate lines for multi-line then expressions

- Put the first line of multi-line `then` expressions (macros, formulas, window functions etc.) on the same line as `then`. Subsequent lines should be indented from `then`.
- The closing parenthesis should be aligned with the function.

```sql
/* Window functions */
case
when customer_id is not null
then row_number() over (
partition by customer_id
order by created_at
)
end as column_name

/* Formulas */
case
when shipping_costs_usd > 0
then shipping_costs_usd
+ shipping_taxes_usd
+ shipping_international_duties_usd
end as column_name
```

<br>

#### Be consistent with spacing after a comma

- When the case statement is preceded by a comma the `end` should be aligned with the comma.

```sql
/* Good */
select
...
, case
when orders.status = 1 then 'Completed'
when orders.status = 2 then 'Processing'
when orders.status = 3 then 'Cancelled'
else 'Failed'
end as column_name

/* Bad */
select
...
, case
when orders.status = 1 then 'Completed'
when orders.status = 2 then 'Processing'
when orders.status = 3 then 'Cancelled'
else 'Failed'
end as column_name

/* Bad */
select
...
, case
when [condition] then [result]
when [condition] then [result]
else [result]
end as column_name
```

<br>

### CTEs

- Where performance permits, CTEs should perform a single, logical unit of work.
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<br>

#### `case` statements:
- You can put a `case` statement all on one line if it only has a single `when` clause and doesn't cause the line's length to be too long.
- For multi-line `case` statements:
- `when`:
- `when` clauses should start on their own line, indented one level more than the `case` statement.
- If a `when` clause has multiple conditions, keep the first condition on the same line as `when` and put subsequent conditions on their own lines.
- If a `when` clause has multiple lines, all its subsequent lines should be indented at least one level more than `when`.
- `then`:
- `then` clauses can go on the same line as a single-line `when` clause if it doesn't cause the line's length to be too long.
- Otherwise, `then` clauses should go on their own line, indented one level more than its associated `when`.
- If a `then` clause has multiple lines, all its subsequent lines should be indented at least one level more than `then`.
- `else`:
- `else` clauses should go on their own line, at the same indentation level as the `when` clauses.
- If an `else` clause has multiple lines, all its subsequent lines should be indented at least one level more than `else`.
- `end`:
- `end` should go on its own line, at the same indentation level as `case`.
- If the `case` starts after a leading comma and space, align `end` with `case` by adding two extra spaces before it.
- If a multi-line `case` statement is within a function call, `case` and `end` should go on their own lines, indented one level more than the function call.
- If using `case <expression>` syntax, keep the expression on the same line as `case`.

```sql
/* Good */
select
case when customers.status_code = 1 then 'Active' else 'Inactive' end as customer_status

/* Bad */
select
case when customers.status_code = 1 and customers.deleted_at is null then 'Active' else 'Inactive' end as customer_status


/* Good */
select
case
when customers.status_code = 1 then 'Active'
else 'Inactive'
end as customer_status
, ...

/* Bad */
select
case when customers.status_code = 1 then 'Active'
else 'Inactive' end as customer_status
, ...


/* Good */
select
...
, case
when customers.status_code = 1
and customers.deleted_at is null
then 'Active'
else 'Inactive'
end as customer_status

/* Bad */
select
...
, case
when customers.status_code = 1 and customers.deleted_at is null
then 'Active'
else 'Inactive'
end as customer_status


/* Good */
select
...
, sum(
case
when customers.status_code = 1
and customers.deleted_at is null
then customers.lifetime_value
else 0
end
) as active_customers_lifetime_value

/* Bad */
select
...
, sum(case
when customers.status_code = 1 and customers.deleted_at is null then customers.lifetime_value
else 0
end) as active_customers_lifetime_value
```

<br>

#### Window functions:
- You can put a window function all on one line if it doesn't cause the line's length to be too long.
- If breaking a window function into multiple lines:
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