# `Lotus.Storage.VariableResolver`
[🔗](https://github.com/elixir-lotus/lotus/blob/v1.3.0/lib/lotus/storage/variable_resolver.ex#L1)

Extracts variable-to-column bindings from SQL queries.

Works out which table column each `{{variable}}` is compared against so
`Lotus.Storage.Query.compile/2` can cast the value to the column's type.
It scans `Lotus.Query.Tokenizer` tokens with a few heuristics; it is not a
SQL parser. A variable it cannot bind falls back to the variable's declared
type, so a miss costs nothing but a cast.

## Supported Patterns

1. **Explicit**: `WHERE users.id = {{user_id}}`
2. **Implicit**: `WHERE id = {{user_id}}` (table from the first `FROM`)
3. **Aliased tables**: `WHERE u.id = {{id}}` (resolves alias `u` to `users`)
4. **Schema-qualified tables**: `FROM public.users` carries `schema: "public"`
5. **Quoted identifiers**: `"Users"."Id"` keeps its case; unquoted names
   are folded to lowercase

A name introduced by a `WITH` clause is not a base table: a column of a
CTE, or of an alias of one, binds with `table: nil`. So does a column of a
subquery in `FROM`.

## Usage

    sql = "SELECT * FROM users WHERE users.id = {{user_id}}"
    VariableResolver.resolve_variables(sql)
    # => [%{variable: "user_id", schema: nil, table: "users", column: "id"}]

# `variable_binding`

```elixir
@type variable_binding() :: %{
  variable: String.t(),
  schema: String.t() | nil,
  table: String.t() | nil,
  column: String.t() | nil
}
```

# `resolve_variables`

```elixir
@spec resolve_variables(String.t(), Lotus.Query.Tokenizer.Profile.t()) :: [
  variable_binding()
]
```

Extract variable bindings from a SQL statement.

Returns one binding per variable, deduplicated by name. An explicit
binding wins over an implicit one, and both win over an unbound
occurrence. The `profile` selects the quoting and comment rules; it
defaults to ANSI SQL.

## Examples

    resolve_variables("SELECT * FROM users WHERE users.id = {{user_id}}")
    # => [%{variable: "user_id", schema: nil, table: "users", column: "id"}]

    resolve_variables("SELECT * FROM users u WHERE u.id = {{user_id}}")
    # => [%{variable: "user_id", schema: nil, table: "users", column: "id"}]

    resolve_variables("SELECT * FROM public.users WHERE id = {{user_id}}")
    # => [%{variable: "user_id", schema: "public", table: "users", column: "id"}]

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*Consult [api-reference.md](api-reference.md) for complete listing*