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Functions Description

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AVG() Returns a column’s average value

COUNT() Returns the number of rows in a column

MAX() Returns a column’s highest value

MIN() Returns a column’s lowest value

SUM() Returns the sum of a column’s values

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AVG()

(jlive)[crashcourse]>SELECT AVG(prod_price) AS avg_price FROM products WHERE vend_id = 1003;

+-----------+

| avg_price |

+-----------+

| 13.212857 |

+-----------+

1 row in set (0.00 sec)

(jlive)[crashcourse]>SELECT AVG(DISTINCT prod_price) AS avg_price FROM products WHERE vend_id = 1003;

+-----------+

| avg_price |

+-----------+

| 15.998000 |

+-----------+

1 row in set (0.00 sec)

COUNT()

(jlive)[crashcourse]>SELECT COUNT(*) AS num_cust FROM customers;

+----------+

| num_cust |

+----------+

|        5 |

+----------+

1 row in set (0.00 sec)

统计表中总共多少行

(jlive)[crashcourse]>SELECT COUNT(cust_email) AS num_cust FROM customers;

+----------+

| num_cust |

+----------+

|        3 |

+----------+

1 row in set (0.00 sec)

统计表中cust_email字段非空的总行数

SUM()

(jlive)[crashcourse]>SELECT SUM(item_price*quantity) AS total_price FROM orderitems WHERE order_num = 20005;

+-------------+

| total_price |

+-------------+

|      149.87 |

+-------------+

1 row in set (0.00 sec)

(jlive)[crashcourse]>SELECT COUNT(*) AS num_items, MIN(prod_price) AS price_min, MAX(prod_price) AS price_max, AVG(prod_price) AS price_avg FROM products;

+-----------+-----------+-----------+-----------+

| num_items | price_min | price_max | price_avg |

+-----------+-----------+-----------+-----------+

|        14 |      2.50 |     55.00 | 16.133571 |

+-----------+-----------+-----------+-----------+

1 row in set (0.00 sec)