查詢數據
下面以一個例子說明如何使用SELECT從單個表中獲取數據。
首先定義數據表,輸入語句如下:
CREATE TABLE fruits
(
f_id char(10) NOT NULL,
s_id INT NOT NULL,
f_name char(255) NOT NULL,
f_price decimal(8,2) NOT NULL,
PRIMARY KEY(f_id)
);
爲了演示如何使用SELECT語句,需要插入如下數據:
INSERT INTO fruits (f_id, s_id, f_name, f_price)
VALUES('a1', 101,'apple',5.2),
('b1',101,'blackberry', 10.2),
('bs1',102,'orange', 11.2),
('bs2',105,'melon',8.2),
('t1',102,'banana', 10.3),
('t2',102,'grape', 5.3),
('o2',103,'coconut', 9.2),
('c0',101,'cherry', 3.2),
('a2',103, 'apricot',2.2),
('l2',104,'lemon', 6.4),
('b2',104,'berry', 7.6),
('m1',106,'mango', 15.6),
('m2',105,'xbabay', 2.6),
('t4',107,'xbababa', 3.6),
('m3',105,'xxtt', 11.6),
('b5',107,'xxxx', 3.6);
使用SELECT語句查詢f_id字段的數據。
SELECT f_id, f_name FROM fruits;
【例1】從fruits表中檢索所有字段的數據,SQL語句如下:
SELECT * FROM fruits;
【例2】查詢fruits表中f_name列所有水果名稱,SQL語句如下:
SELECT f_name FROM fruits;
該語句使用SELECT聲明從fruits表中獲取名稱爲f_name字段下的所有水果名稱,指定字段的名稱緊跟在SELECT關鍵字之後,查詢結果如下:
SELECT f_name FROM fruits;
【例3】例如,從fruits表中獲取f_name和f_price兩列,SQL語句如下:
SELECT f_name, f_price FROM fruits;
該語句使用SELECT聲明從fruits表中獲取名稱爲f_name和f_price兩個字段下的所有水果名稱和價格,兩個字段之間用逗號分隔開,查詢結果如下:
SELECT f_name, f_price FROM fruits;
【例4】查詢價格爲10.2元的水果的名稱,SQL語句如下:
SELECT f_name, f_price
FROM fruits
WHERE f_price = 10.2;
【例5】查找名稱爲“apple”的水果的價格,SQL語句如下:
SELECT f_name, f_price
FROM fruits
WHERE f_name = 'apple';
【例6】查詢價格小於10的水果的名稱,SQL語句如下:
SELECT f_name, f_price
FROM fruits
WHERE f_price < 10;
【例7】s_id爲101和102的記錄,SQL語句如下:
SELECT s_id,f_name, f_price
FROM fruits
WHERE s_id IN (101,102)
ORDER BY f_name;
【例8】查詢所有s_id不等於101也不等於102的記錄,SQL語句如下:
SELECT s_id,f_name, f_price
FROM fruits
WHERE s_id NOT IN (101,102)
ORDER BY f_name;
【例9】查詢價格在2.00元到10.20元之間的水果名稱和價格,SQL語句如下:
SELECT f_name, f_price FROM fruits
WHERE f_price BETWEEN 2.00 AND 10.20;
查詢結果如下:
SELECT f_name, f_price
FROM fruits
WHERE f_price BETWEEN 2.00 AND 10.20;
【例10】查詢價格在2.00元到10.20元之外的水果名稱和價格,SQL語句如下:
SELECT f_name, f_price
FROM fruits
WHERE f_price NOT BETWEEN 2.00 AND 10.20;
【例11】查找所有以’b’字母開頭的水果,SQL語句如下:
SELECT f_id, f_name
FROM fruits
WHERE f_name LIKE 'b%';
【例12】在fruits表中,查詢f_name中包含字母’g’的記錄,SQL語句如下:
SELECT f_id, f_name
FROM fruits
WHERE f_name LIKE '%g%';
【例13】查詢以’b’開頭,並以’y’結尾的水果的名稱,SQL語句如下:
SELECT f_name
FROM fruits
WHERE f_name LIKE 'b%y';
【例14】在fruits表中,查詢以字母’y’結尾,且’y’前面只有4個字母的記錄,SQL語句如下:
SELECT f_id, f_name
FROM fruits
WHERE f_name LIKE '----y';
下面,在數據庫中創建數據表customers,該表中包含了本章中需要用到的數據。
CREATE TABLE customers
(
c_id int NOT NULL AUTO_INCREMENT,
c_name char(50) NOT NULL,
c_address char(50) NULL,
c_city char(50) NULL,
c_zip char(10) NULL,
c_contact char(50) NULL,
c_email char(255) NULL,
PRIMARY KEY (c_id)
);
爲了演示需要插入數據,請讀者插入執行以下語句。
INSERT INTO customers(c_id, c_name, c_address, c_city,
c_zip, c_contact, c_email)
VALUES(10001, 'RedHook', '200 Street ', 'Tianjin',
'300000', 'LiMing', '[email protected]'),
(10002, 'Stars', '333 Fromage Lane',
'Dalian', '116000', 'Zhangbo','[email protected]'),
(10003, 'Netbhood', '1 Sunny Place', 'Qingdao', '266000',
'LuoCong', NULL),
(10004, 'JOTO', '829 Riverside Drive', 'Haikou',
'570000', 'YangShan', '[email protected]');
SELECT COUNT(*) AS cust_num FROM customers;
【例15】查詢customers表中c_email爲空的記錄的c_id、c_name和c_email字段值,SQL語句如下:
SELECT c_id, c_name,c_email FROM customers WHERE c_email IS NULL;
查詢結果如下:
SELECT c_id, c_name,c_email FROM customers WHERE c_email IS NULL;
【例16】查詢customers表中c_email不爲空的記錄的c_id、c_name和c_email字段值,SQL語句如下:
SELECT c_id, c_name,c_email FROM customers WHERE c_email IS NOT NULL;
【例17】在fruits表中查詢s_id = 101,並且f_price大於等於5的水果價格和名稱,SQL語句如下:
SELECT f_id, f_price, f_name FROM fruits WHERE s_id = '101' AND f_price >=5;
【例18】在fruits表中查詢s_id = 101或者102,且f_price大於5,並且f_name=‘apple’的水果價格和名稱,SQL語句如下:
SELECT f_id, f_price, f_name FROM fruits
WHERE s_id IN('101', '102') AND f_price >= 5 AND f_name = 'apple';
【例19】查詢s_id=101或者s_id=102的水果供應商的f_price和f_name,SQL語句如下:
SELECT s_id,f_name, f_price
FROM fruits
WHERE s_id = 101 OR s_id = 102;
查詢結果如下:
SELECT s_id,f_name, f_price
FROM fruits
WHERE s_id = 101 OR s_id = 102;
【例20】查詢s_id=101或者s_id=102的水果供應商的f_price和f_name,SQL語句如下:
SELECT s_id,f_name, f_price
FROM fruits WHERE s_id IN(101,102);
查詢結果如下:
SELECT s_id,f_name, f_price
FROM fruits
WHERE s_id IN(101,102);
【例21】查詢fruits表中s_id字段的值,返回s_id字段值且不得重複,SQL語句如下:
SELECT DISTINCT s_id FROM fruits;
【例22】查詢fruits表的f_name字段值,並對其進行排序,SQL語句如下:
SELECT f_name
FROM fruits ORDER BY f_name;
【例23】查詢fruits表中的f_name和f_price字段,先按f_name排序,再按f_price排序,SQL語句如下:
SELECT f_name, f_price
FROM fruits ORDER BY f_name, f_price;
【例24】查詢fruits表中的f_name和f_price字段,對結果按f_price降序方式排序,SQL語句如下:
SELECT f_name, f_price
FROM fruits ORDER BY f_price DESC;
【例25】查詢fruits表,先按f_price降序排序,再按f_name字段升序排序,SQL語句如下:
SELECT f_price, f_name
FROM fruits ORDER BY f_price DESC, f_name;
【例26】根據s_id對fruits表中的數據進行分組,SQL語句如下:
SELECT s_id, COUNT(*) AS Total
FROM fruits GROUP BY s_id;
【例27】根據s_id對fruits表中的數據進行分組,將每個供應商的水果名稱顯示出來,SQL語句如下:
SELECT s_id, GROUP_CONCAT(f_name) AS Names
FROM fruits GROUP BY s_id;
【例28】根據s_id對fruits表中的數據進行分組,並顯示水果種類大於1的分組信息,SQL語句如下:
SELECT s_id, GROUP_CONCAT(f_name) AS Names
FROM fruits
GROUP BY s_id HAVING COUNT(f_name) > 1;
【例29】根據s_id對fruits表中的數據進行分組,並顯示記錄數量,SQL語句如下:
SELECT s_id, COUNT(*) AS Total
FROM fruits
GROUP BY s_id WITH ROLLUP;
【例30】根據s_id和f_name字段對fruits表中的數據進行分組, SQL語句如下,
SELECT * FROM fruits group by s_id,f_name;
爲了演示效果,首先創建數據表,SQL語句如下:
CREATE TABLE orderitems
(
o_num int NOT NULL,
o_item int NOT NULL,
f_id char(10) NOT NULL,
quantity int NOT NULL,
item_price decimal(8,2) NOT NULL,
PRIMARY KEY (o_num,o_item)
) ;
然後插入演示數據。SQL語句如下:
INSERT INTO orderitems(o_num, o_item, f_id, quantity, item_price)
VALUES(30001, 1, 'a1', 10, 5.2),
(30001, 2, 'b2', 3, 7.6),
(30001, 3, 'bs1', 5, 11.2),
(30001, 4, 'bs2', 15, 9.2),
(30002, 1, 'b3', 2, 20.0),
(30003, 1, 'c0', 100, 10),
(30004, 1, 'o2', 50, 2.50),
(30005, 1, 'c0', 5, 10),
(30005, 2, 'b1', 10, 8.99),
(30005, 3, 'a2', 10, 2.2),
(30005, 4, 'm1', 5, 14.99);
【例31】查詢訂單價格大於100的訂單號和總訂單價格,SQL語句如下:
SELECT o_num, SUM(quantity * item_price) AS orderTotal
FROM orderitems
GROUP BY o_num
HAVING SUM(quantity*item_price) >= 100;
可以看到,返回的結果中orderTotal列的總訂單價格並沒有按照一定順序顯示,接下來,使用ORDER BY關鍵字按總訂單價格排序顯示結果,SQL語句如下:
SELECT o_num, SUM(quantity * item_price) AS orderTotal
FROM orderitems
GROUP BY o_num
HAVING SUM(quantity*item_price) >= 100
ORDER BY orderTotal;
【例32】顯示fruits表查詢結果的前4行,SQL語句如下:
SELECT * From fruits LIMIT 4;
【例33】在fruits表中,使用LIMIT子句,返回從第5個記錄開始的,行數長度爲3的記錄,SQL語句如下:
SELECT * From fruits LIMIT 4, 3;
【例34】查詢customers表中總的行數,SQL語句如下:
SELECT COUNT(*) AS cust_num
FROM customers;
【例35】查詢customers表中有電子郵箱的顧客的總數,SQL語句如下:
SELECT COUNT(c_email) AS email_num
FROM customers;
【例36】在orderitems表中,使用COUNT()函數統計不同訂單號中訂購的水果種類,SQL語句如下:
SELECT o_num, COUNT(f_id)
FROM orderitems
GROUP BY o_num;
【例37】在orderitems表中查詢30005號訂單一共購買的水果總量,SQL語句如下:
SELECT SUM(quantity) AS items_total
FROM orderitems
WHERE o_num = 30005;
【例38】在orderitems表中,使用SUM()函數統計不同訂單號中訂購的水果總量,SQL語句如下:
SELECT o_num, SUM(quantity) AS items_total
FROM orderitems
GROUP BY o_num;
【例39】在fruits表中,查詢s_id=103的供應商的水果價格的平均值,SQL語句如下:
SELECT AVG(f_price) AS avg_price
FROM fruits
WHERE s_id = 103;
【例40】在fruits表中,查詢每一個供應商的水果價格的平均值,SQL語句如下:
SELECT s_id,AVG(f_price) AS avg_price
FROM fruits
GROUP BY s_id;
【例41】在fruits表中查找市場上價格最高的水果,SQL語句如下:
SELECT MAX(f_price) AS max_price FROM fruits;
【例42】在fruits表中查找不同供應商提供的價格最高的水果,SQL語句如下:
SELECT s_id, MAX(f_price) AS max_price
FROM fruits
GROUP BY s_id;
【例43】在fruits表中查找f_name的最大值,SQL語句如下:
SELECT MAX(f_name) FROM fruits;
【例44】在fruits表中查找市場上價格最低的水果,SQL語句如下:
SELECT MIN(f_price) AS min_price FROM fruits;
【例45】在fruits表中查找不同供應商提供的價格最低的水果,SQL語句如下:
SELECT s_id, MIN(f_price) AS min_price
FROM fruits
GROUP BY s_id;
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