docs: add SQL style guide

sql-style-guide.md

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+# SQL Style Guide
+
+This guide establishes the standards for SQL in Boundary.
+
+## Purpose
+
+These guidelines are designed to make our SQL more readable, maintainable, and greppable.
+
+## Rules
+
+### Formatting
+
+- No tabs. 2 spaces per indent.
+- No trailing whitespace.
+- No more than two blank lines between statements.
+- No empty lines in the middle of a single statement.
+- Do not SHOUTCASE or "Sentence case" SQL keywords (e.g., use `select`, not `SELECT` or `Select`).
+
+### Data Definition Statements
+
+Keep the object of `create`, `alter`, `drop`, and `comment on` statements on the
+same line as the statement.
+
+### Tables
+
+```sql
+create table imaginary_basket (
+  public_id wt_public_id primary key,
+  store_id wt_public_id not null
+    constraint imaginary_store_fkey
+      references imaginary_store (public_id)
+      on delete cascade
+      on update cascade,
+  basket_name text not null
+    constraint basket_name_must_not_be_empty
+      check(
+        length(trim(basket_name)) > 0
+      ),
+  basket_type text not null default 'fruit_basket'
+    constraint basket_type_enm_fkey
+      references basket_type_enm (name)
+      on delete restrict
+      on update cascade,
+  constraint imaginary_basket_store_id_basket_name_uq
+    unique(store_id, basket_name)
+);
+comment on table imaginary_basket is
+  'imaginary_basket is a table where each row is a resource that represents an imaginary shopping basket.';
+```
+
+For `create table` statements:
+
+- Keep the `create table` statement and the name of the table on the same line.
+- For columns, keep the column name, column type, `not null` (if applicable),
+  and default value (if applicable) on the same line.
+- Put the `primary key` declaration at the start of the `create table` statement.
+- If the `primary key` is a single column, put the `primary key` declaration on
+  the same line as the column name and column type.
+- Put constraints for a single column on a new line indented under the column declaration.
+
+**Constraints**:
+- Name all constraints.
+- Give `check` constraints a name that describes what rule the constraint is enforcing.
+- The naming pattern for foreign key constraints is `reftable_fkey` where
+  `reftable` is the name of the referenced table.
+- The naming pattern for unique constraints is `tablename_col1_colx_uq` where
+  `tablename` is the name of the table and `col1_colx` is the name of each
+  column in the unique constraint.
+
+### Functions
+
+```sql
+create or replace function colorize_basket(basket_id wt_public_id, basket_color text) returns void
+as $$
+begin
+  ....
+end;
+$$ language plpgsql;
+comment on function colorize_basket is
+  'colorize_basket is a function ....';
+```
+
+For `create function` and `create or replace function` statements:
+
+- Keep the `create function` statement, function name, function parameters and `returns` statement on the same line.
+
+### Triggers
+
+```sql
+create trigger update_version_column after update of version, termination_reason, key_id, tofu_token, server_id, server_type on session
+  for each row execute procedure update_version_column();
+```
+
+For `create trigger` statements:
+- Keep the `create trigger` statement, trigger name, `before | after`, event
+  name, and `on table_name` statement on the same line.
+- Put the remaining statements on a new indented line.
+
+### Domains
+
+```sql
+create domain wt_private_id as text not null
+check(
+  length(trim(value)) > 10
+);
+comment on domain wt_private_id is
+  'Random ID generated with github.com/hashicorp/go-secure-stdlib/base62';
+```
+
+For `create domain` statements:
+- Keep the `create domain` statement, domain name, `as data_type`, `not null`
+  (if applicable), and default value (if applicable) on the same line.
+- Put check constraints on new lines.
+
+### `comment on` statements
+
+For `comment on` statements:
+
+- Keep the `comment on` statement, object type, object name, and `is`
+  statement on the same line.
+- Put `comment on` statements directly below the database object the comment is on.
+- Do not put any blank lines between the `comment on` statement and the database
+  object declaration block that comment is for.
+- Put the text of the `comment on` a new indented line.