sumSimpleState

CREATE TABLE vote_aggregates
(
    post_id UInt32,
    upvotes SimpleAggregateFunction(sum, UInt64),
    downvotes SimpleAggregateFunction(sum, UInt64),
    score SimpleAggregateFunction(sum, Int64)
)
ENGINE = AggregatingMergeTree()
ORDER BY post_id;

CREATE MATERIALIZED VIEW mv_vote_processor TO vote_aggregates
AS
SELECT
  post_id,
  -- sum状態の初期値（upvoteの場合は1、それ以外は0）
  toUInt64(vote_type = 'upvote') AS upvotes,
  -- sum状態の初期値（downvoteの場合は1、それ以外は0）
  toUInt64(vote_type = 'downvote') AS downvotes,
  -- sum状態の初期値（upvoteの場合は1、downvoteの場合は-1）
  toInt64(vote_type) AS score
FROM raw_votes;

INSERT INTO raw_votes VALUES
    (1, 'upvote'),
    (1, 'upvote'),
    (1, 'downvote'),
    (2, 'upvote'),
    (2, 'downvote'),
    (3, 'downvote');

SELECT
  post_id,
  sum(upvotes) AS total_upvotes,
  sum(downvotes) AS total_downvotes,
  sum(score) AS total_score
FROM vote_aggregates -- ターゲットテーブルにクエリを実行する
GROUP BY post_id
ORDER BY post_id ASC;

┌─post_id─┬─total_upvotes─┬─total_downvotes─┬─total_score─┐
│       1 │             2 │               1 │           1 │
│       2 │             1 │               1 │           0 │
│       3 │             0 │               1 │          -1 │
└─────────┴───────────────┴─────────────────┴─────────────┘