Mysql基礎(十二):sql語句執行步驟詳解(一)準備工作
阿新 • • 發佈:2020-11-20
準備工作
先來一段虛擬碼,首先你能看懂麼?
SELECT DISTINCT <select_list> FROM <left_table> <join_type> JOIN <right_table> ON <join_condition> WHERE <where_condition> GROUP BY <group_by_list> HAVING <having_condition> ORDER BY <order_by_condition>LIMIT <limit_number>
繼續做以下的前期準備工作:
新建一個測試資料庫TestDB;
create database TestDB;
建立測試表table1和table2;
CREATE TABLE table1 ( customer_id VARCHAR(10) NOT NULL, city VARCHAR(10) NOT NULL, PRIMARY KEY(customer_id) )ENGINE=INNODB DEFAULT CHARSET=UTF8; CREATE TABLE table2 ( order_idINT NOT NULL auto_increment, customer_id VARCHAR(10), PRIMARY KEY(order_id) )ENGINE=INNODB DEFAULT CHARSET=UTF8;
插入測試資料;
INSERT INTO table1(customer_id,city) VALUES('163','hangzhou'); INSERT INTO table1(customer_id,city) VALUES('9you','shanghai'); INSERT INTO table1(customer_id,city) VALUES('tx','hangzhou'); INSERT INTO table1(customer_id,city) VALUES('baidu','hangzhou'); INSERT INTO table2(customer_id) VALUES('163'); INSERT INTO table2(customer_id) VALUES('163'); INSERT INTO table2(customer_id) VALUES('9you'); INSERT INTO table2(customer_id) VALUES('9you'); INSERT INTO table2(customer_id) VALUES('9you'); INSERT INTO table2(customer_id) VALUES('tx'); INSERT INTO table2(customer_id) VALUES(NULL);
準備工作做完以後,table1和table2看起來應該像下面這樣:
mysql> select * from table1; +-------------+----------+ | customer_id | city | +-------------+----------+ | 163 | hangzhou | | 9you | shanghai | | baidu | hangzhou | | tx | hangzhou | +-------------+----------+ 4 rows in set (0.00 sec)
mysql> select * from table2; +----------+-------------+ | order_id | customer_id | +----------+-------------+ | 1 | 163 | | 2 | 163 | | 3 | 9you | | 4 | 9you | | 5 | 9you | | 6 | tx | | 7 | NULL | +----------+-------------+ 7 rows in set (0.00 sec)
準備SQL邏輯查詢測試語句
SELECT a.customer_id, COUNT(b.order_id) as total_orders FROM table1 AS a LEFT JOIN table2 AS b ON a.customer_id = b.customer_id WHERE a.city = 'hangzhou' GROUP BY a.customer_id HAVING count(b.order_id) < 2 ORDER BY total_orders DESC;
使用上述SQL查詢語句來獲得來自杭州,並且訂單數少於2的客戶。