DECLARE keyword

DECLARE specifies a series of variable bindings used throughout your query.

This syntax is supported within SELECT queries.

Syntax

Mechanics

The DECLARE keyword comes before the SELECT clause in your query:

DECLARE
    @x := 5
SELECT @x;

Use the variable binding operator := (walrus) to associate expressions to names.

:::tip

It is easy to accidentally omit the : when writing variable binding expressions.

Don't confuse the := operator with a simple equality =!

You should see an error message like this:

expected variable assignment operator :=

:::

The above example declares a single binding, which states that the variable @x is replaced with the constant integer 5.

The variables are resolved at parse-time, meaning that the variable is no longer present when the query is compiled.

So the above example reduces to this simple query:

SELECT 5;

Multiple bindings

To declare multiple variables, set the bind expressions with commas ,:

DECLARE
    @x := 5,
    @y := 2
SELECT @x + @y;

Variables as functions

A variable need not be just a constant. It could also be a function call, and variables with function values can be nested:

DECLARE
  @today := today(),
  @start := interval_start(@today),
  @end := interval_end(@today)
SELECT @today = interval(@start, @end);

Declarations in subqueries

Declarations made in parent queries are available in subqueries.

DECLARE
    @x := 5
SELECT y FROM (
    SELECT @x AS y
);

Shadowing

If a subquery declares a variable of the same name, then the variable is shadowed and takes on the new value.

However, any queries above this subquery are unaffected - the variable bind is not globally mutated.

DECLARE
    @x := 5
SELECT @x + y FROM (
    DECLARE @x := 10
    SELECT @x AS y
);

Declarations as subqueries

Declarations themselves can be subqueries.

We suggest that this is not overused, as removing the subquery definition from its execution location may make queries harder to debug.

Nevertheless, it is possible to define a variable as a subquery:

DECLARE
    @subquery := (SELECT timestamp FROM trades)
SELECT * FROM @subquery;

You can even use already-declared variables to define your subquery variable:

DECLARE
    @timestamp := timestamp,
    @symbol := symbol,
    @subquery := (SELECT @timestamp, @symbol FROM trades)
SELECT * FROM @subquery;

Declarations in CTEs

Naturally, DECLARE also works with CTEs:

DECLARE
  @x := 5
WITH first AS (
  DECLARE @x := 10
  SELECT @x as a -- a = 10
),
second AS (
  DECLARE @y := 4
  SELECT
    @x + @y as b, -- b = 5 + 4 = 9
    a -- a = 10
    FROM first
)
SELECT a, b
FROM second;

Bind variables

DECLARE syntax will work with prepared statements over PG Wire, so long as the client library does not perform syntax validation that rejects the DECLARE syntax:

DECLARE @x := ?, @y := ?
SELECT @x::int + @y::int;

-- Then bind the following values: (1, 2)

This can be useful to minimise repeated bind variables.

For example, rather than passing the same value to multiple positional arguments, you could instead use a declared variable and send a single bind variable:

-- instead of this:
SELECT ? as name, id FROM users WHERE name = ?;

-- do this:
DECLARE @name := ?
SELECT @name as name, id FROM users WHERE name = @name;

Or for repeating columns:

DECLARE
    @col = ?,
    @symbol = ?
SELECT avg(@col), min(@col), max(@col)
FROM trades
WHERE symbol = @symbol;

Limitations

Most basic expressions are supported, and we provide examples later in this document.

We suggest you use variables to simplify repeated constants within your code, and minimise how many places you need to update the constant.

Disallowed expressions

However, not all expressions are supported. The following are explicitly disallowed:

Bracket lists

DECLARE
    @symbols := ('BTC-USDT', 'ETH-USDT')
SELECT timestamp, price, symbol
FROM trades
WHERE symbol IN @symbols;

-- error: unexpected bind expression - bracket lists not supported

SQL statement fragments

DECLARE
    @x := FROM trades
SELECT 5 @x;

-- table and column names that are SQL keywords have to be enclosed in double quotes, such as "FROM"```

Language client support

Some language SQL clients do not allow identifiers to be passed as if it was a normal value. One example is psycopg. In this case, you should use an alternate API to splice in identifiers, for example:

cur.execute(
    sql.SQL("""
        DECLARE @col := {}
        SELECT max(@col), min(@col), avg(price)
        FROM btc_trades;
    """).format(sql.Identifier('price')))

Examples

SAMPLE BY

DECLARE
    @period := 1m,
    @window := '2024-11-25',
    @symbol := 'ETH-USDT'
SELECT
   timestamp, symbol, side, sum(amount) as volume
FROM trades
WHERE side = 'sell'
AND timestamp IN @window
AND symbol = @symbol
SAMPLE BY @period
FILL(NULL);

INSERT INTO SELECT

INSERT INTO trades (timestamp, symbol)
SELECT * FROM
(
    DECLARE
        @x := now(),
        @y := 'ETH-USDT'
    SELECT @x as timestamp, @y as symbol
);

CREATE TABLE AS SELECT

CREATE TABLE trades AS (
    DECLARE
        @x := now(),
        @y := 'ETH-USDT'
    SELECT @x as timestamp, @y as symbol, 123 as price
);