CREATE TABLE tenant_memberships (
    id          BIGSERIAL PRIMARY KEY,
    tenant_id   BIGINT NOT NULL REFERENCES tenants(id),
    user_id     BIGINT NOT NULL REFERENCES users(id),
    tenant_role VARCHAR(20) NOT NULL,
    created_at  TIMESTAMPTZ NOT NULL DEFAULT NOW(),
    updated_at  TIMESTAMPTZ NOT NULL DEFAULT NOW(),
    deleted_at  TIMESTAMPTZ
);
CREATE UNIQUE INDEX uk_tenant_user_active ON tenant_memberships(tenant_id, user_id) WHERE deleted_at IS NULL;