SQL笔记 - 高阶

• 返回一定数目记录

SELECT TOP number ( number percent) column FROM table
SELECT column FROM table LIMIT number /MySQL
SELECT column FROM table WHERE ROWNUM <= number

• 表/列 别名

SELECT t.column FROM table AS t
SELECT column AS c FROM table

• 不同表关联
  主键 —— 值的唯一性

SELECT table1.column1, table2.column2
FROM table1, table2
WHERE table1.column-n = table2.column-m
/
SELECT table1.column1, table2.column2
FROM table1 JOIN table2
ON table1.column-n = table2.column-m

/JOIN for example/

• 不同表合并

SELECT column1 FROM table1
UNION (ALL /允许重复值/)
SELECT column2 FROM table2
(column1与column2的列数相同，合并项处于同一顺序上)

• 创建备份表

SELECT column INTO new_table (IN database) FROM old_table

• 自动创建主键值，新增行时无需为主键赋值

AUTO_INCREMENT (= number) /number = 起始值
IDENTITY (n, m) /n起始，m递增

CREAT SEQUENCE sequence
MINVALUE n
START WITH n
INCREMENT BY m
CACHE x

INSERT INTO table (column) VALUES (sequence.nextval)

• 视图

CREATE VIEW view AS SELECT column FROM table WHERE condition
REPLACE VIEW view AS SELECT column FROM table WHERE condition
DROP VIEW view

• 空值测试

SELECT column FROM table WHERE column IS NULL
SELECT column FROM table WHERE column IS NOT NULL