#  []表示里面內(nèi)容可選，使用時(shí)不帶[]本身
CREATE TABLE [IF NOT EXISTS] table_name (
    column1 datatype [constraints],
    column2 datatype [constraints],
    ...
    [table_constraints]
) [ENGINE=storage_engine] [DEFAULT CHARSET=charset];

-- 1：創(chuàng)建數(shù)據(jù)庫
CREATE DATABASE sql_stu;
-- 2：使用數(shù)據(jù)庫
USE sql_stu;
-- 3：創(chuàng)建表
CREATE TABLE IF NOT EXISTS table_stu (
    id INT PRIMARY KEY AUTO_INCREMENT,
    username VARCHAR(50) NOT NULL UNIQUE,
    age INT DEFAULT 18,
    create_time TIMESTAMP DEFAULT CURRENT_TIMESTAMP
);
-- 使用 DESCRIBE 表名 或 SHOW CREATE TABLE 表名 查看表
DESCRIBE table_name;-- 或簡寫為：DESC table_name;

ALTER TABLE 表名
    操作1,
    ...;

ALTER TABLE employees 
#添加列
ADD COLUMN 列名 數(shù)據(jù)類型 AFTER 某個(gè)列;
#修改列數(shù)據(jù)類型與重命名
MODIFY COLUMN 列名 數(shù)據(jù)類型;
CHANGE COLUMN 老列名 新列名 數(shù)據(jù)類型;
#刪除列
DROP COLUMN 列名;
#添加約束
ADD PRIMARY KEY (emp_id);
---
外鍵？
#刪約束
DROP FOREIGN KEY 外鍵名;
DROP PRIMARY KEY;

#僅復(fù)制結(jié)構(gòu)
CREATE TABLE 新表 LIKE 源表;
#復(fù)制結(jié)構(gòu)+數(shù)據(jù)
CREATE TABLE 新表 AS 
SELECT * FROM 源表;
#選擇復(fù)制結(jié)構(gòu)+數(shù)據(jù)
CREATE TABLE 新表 AS 
SELECT 列1，列2，列3 
FROM 源表
WHERE 條件;

-- 創(chuàng)建表
CREATE TABLE stuinfo (
    stuid INT PRIMARY KEY,
    stuname VARCHAR(20) UNIQUE NOT NULL,
    stugender CHAR(1) DEFAULT '男',
    email VARCHAR(20) NOT NULL,
    age INT,
    majorid INT,
    CONSTRAINT fk_stuinfo_major FOREIGN KEY (majorid) REFERENCES major(id)
);
-- 修改表結(jié)構(gòu)
ALTER TABLE stuinfo RENAME TO stuinfo1;  -- 重命名表
ALTER TABLE stuinfo1 ADD COLUMN borndate TIMESTAMP NOT NULL;  -- 添加列
ALTER TABLE stuinfo1 CHANGE COLUMN borndate birthday DATETIME;  -- 修改列名
ALTER TABLE stuinfo1 MODIFY COLUMN birthday DATE;  -- 修改數(shù)據(jù)類型
ALTER TABLE stuinfo1 DROP COLUMN age;  -- 刪除列
-- 復(fù)制表
CREATE TABLE newTable LIKE stuinfo;  -- 僅復(fù)制結(jié)構(gòu)
CREATE TABLE emp_copy SELECT * FROM employees;  -- 復(fù)制結(jié)構(gòu)和數(shù)據(jù)

SELECT [DISTINCT] 列1, 列2, ...  --選擇需要顯示的列
FROM 表名    				     --首先確定數(shù)據(jù)來源
[WHERE 條件]     	 		      --對(duì)原始數(shù)據(jù)進(jìn)行篩選
[GROUP BY 分組列]   			 --對(duì)篩選后的數(shù)據(jù)分組
[HAVING 分組條件]   		    --對(duì)分組后的數(shù)據(jù)進(jìn)行篩選
[ORDER BY 排序列 [ASC|DESC]]     --對(duì)結(jié)果進(jìn)行排序
[LIMIT [偏移量,] 行數(shù)];          --限制返回結(jié)果數(shù)量

-- 查詢所有列
SELECT * FROM users;
-- 查詢指定列
SELECT id, ... FROM users;
-- 列別名
SELECT id AS user_id, username AS name FROM users;--（AS可省略）
-- 去重查詢（DISTINCT）
SELECT DISTINCT country FROM customers;
-- 連接字段 CONCAT
SELECT CONCAT(last_name, first_name) AS 姓名 FROM employees;

-- 單表查詢
SELECT * FROM products;
-- 多表連接查詢（內(nèi)連接）
SELECT * 
FROM orders
INNER JOIN users ON orders.user_id = users.id;
SELECT IFNULL(commission_pct, 0.00) AS 獎(jiǎng)金, commission_pct
FROM employees;

/* 語法結(jié)構(gòu)：
   SELECT 查詢列表
   FROM 表名
   WHERE 篩選條件   */
-- 比較運(yùn)算符：> < = != <> >= <=
SELECT * FROM employees WHERE salary > 12000;
-- 邏輯運(yùn)算符：AND OR NOT
SELECT last_name, salary
FROM employees
WHERE salary >= 10000 AND salary <= 20000;
-- 范圍查詢 BETWEEN AND，或者使用 salary > 10000 AND salary < 20000
SELECT * 
FROM employees
WHERE salary BETWEEN 10000 AND 20000;

-- IN 查詢，IN ()任意一個(gè)返回TRUE(1)
SELECT last_name, job_id
FROM employees
WHERE job_id IN ('AD_PRES', 'IT_PROG', 'PU_CLERK');
-- 是 NULL 值判斷
SELECT last_name, commission_pct
FROM employees
WHERE commission_pct IS NULL;  -- IS NOT NULL
-- 安全等于 <=> (可判斷NULL和普通值)
SELECT last_name, commission_pct
FROM employees
WHERE commission_pct <=> NULL;

-- 基本通配符：% 匹配任意個(gè)字符, _ 匹配單個(gè)字符
SELECT last_name
FROM employees
WHERE last_name LIKE '_a_%' ESCAPE 'a';  -- 第二個(gè)字符為_
-- 使用 \ 轉(zhuǎn)義
...同上
WHERE last_name LIKE '_\_%';  -- 使用 \ 轉(zhuǎn)義

-- 統(tǒng)計(jì)每個(gè)部門的員工數(shù)
SELECT department, COUNT(*) AS employee_count
FROM employees
GROUP BY department;
-- 分組后過濾（HAVING）
SELECT category, AVG(price) AS avg_price
FROM products
GROUP BY category
HAVING avg_price > 200;  -- 只返回平均價(jià)格>200的分組

-- 基本排序
SELECT * FROM employees ORDER BY salary DESC;  --ASC 升序(默認(rèn))， DESC （drop降序）
-- 多字段排序
SELECT *
FROM employees
ORDER BY salary DESC, employee_id ASC;
-- 按表達(dá)式和函數(shù)結(jié)果多字段排序
SELECT 
    LENGTH(last_name)  字符長度,  
    salary * 12 * (1 + IFNULL(commission_pct, 0))  年薪
FROM employees
ORDER BY 年薪 DESC, 字符長度 DESC;  -- 按年薪和字符長度排序

SELECT department, AVG(salary) AS avg_salary
FROM employees
GROUP BY department
HAVING AVG(salary) > 6000;

-- 提取第2頁，每頁10條           偏移量 每頁行數(shù)
 SELECT  *  FROM  products LIMIT 10  ,  10;  -- 等價(jià)于 OFFSET 10 LIMIT 10
-- 帶排序的分頁
SELECT *
FROM employees
ORDER BY salary DESC
LIMIT 20, 10;  -- 第3頁

select ename, sal,dname from EMP, DEPT where EMP.deptno=DEPT.deptno andDEPT.deptno = 10;

SELECT *
FROM products
WHERE (category, price) IN (SELECT category, price FROM products WHERE category = 'Electronics');

-- from子查詢 (返回臨時(shí)表)
SELECT dep_ag.department_id, dep_ag.ag, j.grade_level
FROM (
    SELECT AVG(salary) ag, department_id
    FROM employees
    GROUP BY department_id
) AS dep_ag
INNER JOIN job_grades j ON dep_ag.ag BETWEEN j.lowest_sal AND j.highest_sal;

-- 基本聯(lián)合
SELECT employee_id FROM employees WHERE salary > 15000
UNION
SELECT employee_id FROM employees WHERE commission_pct > 0.2;
-- UNION ALL (保留重復(fù)記錄)
SELECT department_id FROM employees
UNION ALL
SELECT department_id FROM departments;

select 字段 from 表1 inner join 表2 on 連接條件 and 其他條件;

--用前面的寫法
select ename,dname from EMP, DEPT where EMp.deptno=DEPT.deptno andename='SMITH'
--用標(biāo)準(zhǔn)的內(nèi)連接寫法
select ename, dname from EMp inner join DEPT on EMP.deptno=DEPT.deptno andename='SMITH':