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CREATE TABLE warehouses (
    id    bigserial   PRIMARY KEY,
    code  varchar(10) NOT NULL UNIQUE,
    city  varchar(80) NOT NULL
);
COMMENT ON TABLE warehouses IS 'Physical locations; code appears on labels';

CREATE TABLE items (
    id       bigserial    PRIMARY KEY,
    sku      varchar(40)  NOT NULL UNIQUE,
    title    varchar(200) NOT NULL,
    weight_g int
);
COMMENT ON COLUMN items.weight_g IS 'Grams; NULL for services and digital goods';

CREATE TABLE stock (
    warehouse_id bigint NOT NULL REFERENCES warehouses(id),
    item_id      bigint NOT NULL REFERENCES items(id),
    on_hand      int    NOT NULL DEFAULT 0,
    reserved     int    NOT NULL DEFAULT 0,
    PRIMARY KEY (warehouse_id, item_id)
);
COMMENT ON TABLE stock IS 'Current quantities; history lives in stock_moves';

CREATE TABLE stock_moves (
    id           bigserial   PRIMARY KEY,
    item_id      bigint      NOT NULL REFERENCES items(id),
    from_wh      bigint      REFERENCES warehouses(id),
    to_wh        bigint      REFERENCES warehouses(id),
    qty          int         NOT NULL,
    moved_at     timestamptz NOT NULL DEFAULT now()
);
COMMENT ON COLUMN stock_moves.from_wh IS 'NULL for inbound deliveries';
COMMENT ON COLUMN stock_moves.to_wh IS 'NULL for write-offs and shipments';