1. DQL（Data Query Language：數據庫查詢語言）知識點概覽

DQL（Data Query Language：數據庫查詢語言）：

用來查詢數據(記錄)，不會對數據進行改變，而是讓數據庫發送結果集給客戶端

概覽：

2.基本語法

select 列名1，列名2…….列名n from 表名 where 條件 group by 列名 having 條件 order by 列名；

這個SQL語句包含了DQL部分所有的查詢知識點。簡單點說，只要記住這句話，查詢部分的語法基本就都掌握了。
下面我們將對各個部分分開進行詳細說明。

3.查詢方法概覽

創建一個數據庫和表格，方便查詢演示。代碼如下：

這里我是使用SQLyog創建的表格，使用cmd命令窗口也可以直接執行下列語句，后面的所有演示都是使用該軟件寫的，方便進行說明和注釋

-- 創建數據庫和表格方便演示使用 CREATE DATABASE queryDemo; -- 創建數據庫queryDemo USE queryDemo; -- 進入queryDemo數據庫 -- 在queryDemo中創建表格demo1Student CREATE TABLE demo1Student(id INT PRIMARY KEY AUTO_INCREMENT,NAME VARCHAR(20),age INT,score INT);-- 插入數據 INSERT INTO demo1Student (NAME,age,score) VALUES ("張三",18,89); INSERT INTO demo1Student (NAME,age,score) VALUES ("李四",23,77); INSERT INTO demo1Student (NAME,age,score) VALUES ("王五",19,69); INSERT INTO demo1Student (NAME,age,score) VALUES ("趙六",18,89); INSERT INTO demo1Student (NAME,age,score) VALUES ("田七",20,92); INSERT INTO demo1Student (NAME,age,score) VALUES ("小明",22,85); INSERT INTO demo1Student (NAME,age,score) VALUES ("小紅",25,97);

4.基礎查詢

代碼演示

-- 1.1查詢所有列： SELECT * FROM demo1Student; -- 1.2查詢指定列： SELECT id,NAME FROM demo1Student; SELECT NAME,age,score FROM demo1Student;

效果圖：

SELECT * FROM demo1Student;

SELECT id,NAME FROM demo1Student;

SELECT NAME,age,score FROM demo1Student;

5.條件查詢

代碼演示

-- 條件查詢 運算符 關鍵字-- 1.運算符 =、!=、<>、<、<=、>、>=；-- 這些都是普通的數學運算符，就就不做說明了， <>：表示不等于SELECT * FROM demo1Student WHERE id=5;SELECT * FROM demo1Student WHERE age!=18;SELECT * FROM demo1Student WHERE age<>18;SELECT * FROM demo!student WHERE score>90;SELECT NAME,age FROM demo1Student WHERE NAME="張三";-- 2.BETWEEN…AND； IN(set)； IS NULL； IS NOT NULL； AND；OR；NOT-- 在...之間 與set集合交集 是否為空 是否為非空 并且 或 非SELECT * FROM demo1Student WHERE id BETWEEN 2 AND 4; -- id在2到4之間的學生-- in(set)：指與數據庫中數據與set的交集，獲得包含有set中數據的記錄（行）SELECT * FROM demo1Student WHERE age IN(25,22,30);SELECT * FROM demo1Student WHERE NAME IN ("小","張三","哈哈");-- 檢驗null， 所以添加一個新數據INSERT INTO demo1student VALUES (8,"小花",20,NULL);SELECT * FROM demo1Student WHERE score IS NULL; -- 分數為空的學生SELECT * FROM demo1Student WHERE score IS NOT NULL; -- 分數不為空的學生SELECT * FROM demo1Student WHERE age=18 AND score>78; SELECT * FROM demo1Student WHERE age=22 OR age=25; SELECT * FROM demo1Student WHERE NOT age=18; -- age不是18歲的學生

運行結果：

---

運算符
SELECT * FROM demo1Student WHERE id=5;

SELECT * FROM demo1Student WHERE age!=18;

SELECT * FROM demo1Student WHERE age<>18;

SELECT * FROM demo1student WHERE score>90;

SELECT NAME,age FROM demo1Student WHERE NAME=”張三”;

---

關鍵字

SELECT * FROM demo1Student WHERE id BETWEEN 2 AND 4; – id在2到4之間的學生

---

– in(set)：指與數據庫中數據與set的交集，獲得包含有set中數據的記錄（行）
SELECT * FROM demo1Student WHERE age IN(25,22,30);

SELECT * FROM demo1Student WHERE NAME IN (“小”,”張三”,”哈哈”);

---

SELECT * FROM demo1Student WHERE score IS NULL; – 分數為空的學生

SELECT * FROM demo1Student WHERE score IS NOT NULL; – 分數不為空的學生

SELECT * FROM demo1Student WHERE age=18 AND score>78;

SELECT * FROM demo1Student WHERE age=22 OR age=25;

SELECT * FROM demo1Student WHERE NOT age=18; – age不是18歲的學生

6.模糊查詢

代碼演示

-- 3.模糊查詢-- _ 任意一個字符SELECT * FROM demo1student WHERE age LIKE "2_";SELECT * FROM demo1student WHERE age LIKE "_8";SELECT * FROM demo1student WHERE NAME LIKE "小_";-- %：任意0~n個字符SELECT * FROM demo1student WHERE age LIKE "%8";SELECT * FROM demo1student WHERE NAME LIKE "%小%";

---

運行結果

– _ 任意一個字符
SELECT * FROM demo1student WHERE age LIKE “2_”;

SELECT * FROM demo1student WHERE age LIKE “_8”;

SELECT * FROM demo1student WHERE NAME LIKE “小_”;

---

– %：任意0~n個字符

SELECT * FROM demo1student WHERE age LIKE “%8”;

SELECT * FROM demo1student WHERE NAME LIKE “%小%”;

7.排序

代碼演示

-- 4.排序order by -- 升序ascSELECT * FROM demo1student WHERE score>80 ORDER BY score ASC;-- 降序descSELECT * FROM demo1student WHERE score>80 ORDER BY score DESC;

---

運行結果

– 升序asc
SELECT * FROM demo1student WHERE score>80 ORDER BY score ASC;

– 降序desc
SELECT * FROM demo1student WHERE score>80 ORDER BY score DESC;

8.其他四種排序見下節

地址：http://blog.csdn.net/baidu_37107022/article/details/72627463

總結

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