MySQL中union和union all区别

create table products
(
    id          int auto_increment
        primary key,
    name        varchar(255)                        not null,
    price       decimal(10, 2)                      not null,
    description text                                null,
    created_at  timestamp default CURRENT_TIMESTAMP null,
    updated_at  timestamp default CURRENT_TIMESTAMP null on update CURRENT_TIMESTAMP,
    type        tinyint                             not null comment '商品类型'
);
create index idx_name_prefix
    on products (name(7));
create index idx_type
    on products (type);
create index products_name_index
    on products (name desc);
create index products_price_index
    on products (price desc);
# 1.执行
CREATE PROCEDURE generate_test_data()
BEGIN
    DECLARE counter INT DEFAULT 1;
    WHILE counter <= 10000 DO
        INSERT INTO products (id, name, price, description, type)
        VALUES (counter, CONCAT('Product ', counter), RAND() * 100, CONCAT('Description for product ', counter), FLOOR(RAND() * 5));
        SET counter = counter + 1;
    END WHILE;
END;
# 2.调用
CALL generate_test_data();

explain
select id, name
from products
where name = 'Product B'
  or price = 5.99;

explain
SELECT *
FROM products
where name = 'Product B'
union all
SELECT *
FROM products
where price = 5.99;

SELECT *
FROM products
where name = 'Product B'
union all
SELECT *
FROM products
where price = 19.99;

SELECT *
FROM products
where name = 'Product B'
union
SELECT *
FROM products
where price = 19.99;