union.md

# Union

现有员工信息表和顾客信息表如下

```sql
create table employee(
    id int primary key auto_increment,
    name varchar(256),
    address varchar(1024),
    dept varchar(64)
    -- ignore more
);

create table customer(
     id int primary key auto_increment,
     name varchar(256),
     address varchar(1024),
     level int
    -- ignore more
)

```

Joe 需要员工和顾客的联系方式（姓名+地址）清单，用于邮寄礼品。这个查询如何写？

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## 答案

```sql
select name, address 
from customer
union
select name, address
from employee
```

## 选项

### A

```sql
select * 
from customer
union
select *
from employee
```

### B

```sql
select * 
from customer
join employee
```

### C

```sql
select * 
from customer
join employee on customer.id = employee.id
```

### D

```sql
select * 
from customer, employee
```

### E

```sql
select name, address 
from customer, employee
```