ALTER TABLE基本語法來添加新的列到現(xiàn)有表如下：

ALTER TABLE table_name ADD column_name datatype;

ALTER TABLE用于DROP COLUMN在現(xiàn)有表的基本語法如下：

ALTER TABLE table_name DROP COLUMN column_name;

ALTER TABLE的基本語法更改列的數(shù)據(jù)類型(DATA TYPE)在表中如下：

ALTER TABLE table_name MODIFY COLUMN column_name datatype;

ALTER TABLE添加NOT NULL約束到一個表的列的基本語法如下：

ALTER TABLE table_name MODIFY column_name datatype NOT NULL;

ALTER TABLE添加唯一約束到表的基本語法如下：

ALTER TABLE table_name ADD CONSTRAINT MyUniqueConstraint UNIQUE(column1, column2...);

ALTER TABLE添加CHECK約束到表的基本語法如下：

ALTER TABLE table_name ADD CONSTRAINT MyUniqueConstraint CHECK (CONDITION);

ALTER TABLE添加PRIMARY KEY約束到表的基本語法如下：

ALTER TABLE table_name ADD CONSTRAINT MyPrimaryKey PRIMARY KEY (column1, column2...);

ALTER TABLE從表中刪除約束的基本語法如下：

ALTER TABLE table_name DROP CONSTRAINT MyUniqueConstraint;

如果使用MySQL，參考代碼如下：

ALTER TABLE table_name DROP INDEX MyUniqueConstraint;

ALTER TABLE從表中刪除主鍵約束的基本語法如下：

ALTER TABLE table_name DROP CONSTRAINT MyPrimaryKey;

如果使用MySQL几苍，代碼如下：

ALTER TABLE table_name DROP PRIMARY KEY;

2.SQL truncate table 清空表
語法
truncate table的基本語法如下：

TRUNCATE TABLE table_name;

3.HAVING子句
允許您指定過濾器的條件翻屈，分組結(jié)果會出現(xiàn)在最終結(jié)果。

WHERE子句所選列的條件妻坝，而由GROUP BY子句條件創(chuàng)建分組的HAVING子句伸眶。

SELECT
FROM
WHERE
GROUP BY
HAVING
ORDER BY

SELECT column1, column2
FROM table1, table2
WHERE [ conditions ]
GROUP BY column1, column2
HAVING [ conditions ]
ORDER BY column1, column2

考慮CUSTOMERS表具有以下記錄：

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
+----+----------+-----+-----------+----------+
下面是示例，這將顯示記錄age大于或等于2的數(shù)據(jù)量：
SQL > SELECT ID, NAME, AGE, ADDRESS, SALARY
FROM CUSTOMERS
GROUP BY age
HAVING COUNT(age) >= 2;
這將產(chǎn)生以下結(jié)果：

+----+--------+-----+---------+---------+
| ID | NAME   | AGE | ADDRESS | SALARY  |
+----+--------+-----+---------+---------+
|  2 | Khilan |  25 | Delhi   | 1500.00 |
+----+--------+-----+---------+---------+