查询中使用算术操作符 
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算术操作符包括：+(加)、-(减)、\*(乘)、/(除)、-(取反)、MOD(取模)。这些操作符可以作用在数值列上。

如"从多个表里查询数据" 页面的示例查询出客户购买的每个商品的数量和价格，其中数量乘以价格就是每类商品的支付总额。所以 select_list可以增加一列 t3.ol_quantity \* t4.i_price item_sum_price 。

```javascript
SELECT t1.c_first, t1.c_last, t1.c_credit, t2.o_ol_cnt, t2.o_entry_d, t3.ol_number, t3.ol_quantity, t4.i_name, t4.i_price, t3.ol_quantity * t4.i_price item_sum_price
FROM cust t1 
    JOIN ordr t2 ON (t1.c_id=t2.o_id AND t1.c_w_id=t2.o_w_id AND t1.c_d_id=t2.o_d_id)
    JOIN ordl t3 ON (t2.o_id=t3.ol_o_id AND t2.o_w_id=t3.ol_w_id AND t2.o_d_id=t3.ol_d_id)
    JOIN item t4 ON (t4.i_id=t3.ol_i_id )
WHERE t1.c_w_id=2 AND t1.c_d_id=5 and t1.c_last LIKE 'CALLY%'
ORDER BY t1.c_id, t2.o_id, t3.ol_number
;
```



查询结果如下：

```unknow
+------------------+----------------+----------+----------+------------+-----------+-------------+--------------------------+---------+----------------+
| c_first          | c_last         | c_credit | o_ol_cnt | o_entry_d  | ol_number | ol_quantity | i_name                   | i_price | item_sum_price |
+------------------+----------------+----------+----------+------------+-----------+-------------+--------------------------+---------+----------------+
| wPS9EgAgztLRvSuZ | CALLYABLEOUGHT | GC       |        7 | 2020-02-15 |         1 |           5 | FJT8fkxaUh2aUbI          |   79.95 |         399.75 |
| wPS9EgAgztLRvSuZ | CALLYABLEOUGHT | GC       |        7 | 2020-02-15 |         2 |           5 | kiMk43vd9HidvmwG8x       |   58.59 |         292.95 |
| wPS9EgAgztLRvSuZ | CALLYABLEOUGHT | GC       |        7 | 2020-02-15 |         3 |           5 | JnJEOLUCjunrKkt4Z1pL     |   85.26 |         426.30 |
| wPS9EgAgztLRvSuZ | CALLYABLEOUGHT | GC       |        7 | 2020-02-15 |         4 |           5 | CrFVAZW3OhyekdDNc2rPH    |   22.30 |         111.50 |
| wPS9EgAgztLRvSuZ | CALLYABLEOUGHT | GC       |        7 | 2020-02-15 |         5 |           5 | fJpsyG11EjWIceJWaB       |   41.39 |         206.95 |
| wPS9EgAgztLRvSuZ | CALLYABLEOUGHT | GC       |        7 | 2020-02-15 |         6 |           5 | shseF8WI1VSPbWfswSsIuNC  |   30.04 |         150.20 |
| wPS9EgAgztLRvSuZ | CALLYABLEOUGHT | GC       |        7 | 2020-02-15 |         7 |           5 | prjdpUDOxRvAn5WiMVoT85B1 |   18.55 |          92.75 |
+------------------+----------------+----------+----------+------------+-----------+-------------+--------------------------+---------+----------------+
7 rows in set (0.01 sec)
```