1. 从条款：
它用于选择将使用 Select、Update 或 Delete 命令操作的数据集。它与 SQL 语句结合使用从源表中操作数据集。我们可以使用 FROM 子句中的子查询从表中检索数据集。
FROM 子句的语法：
SELECT *
FROM TABLE_NAME; 2. WHERE 条款：
它用于对选定的数据集或源数据应用任何条件。源数据可以是单个表，也可以是连接多个表的结果。它返回满足 WHERE 子句中提到的条件的那些数据集实例。可以应用条件使用各种比较或逻辑运算符，如 –
AND、OR、IN、NOT IN、BETWEEN、等于、不等于等。
WHERE 子句的语法：
SELECT * FROM TABLE_NAME
WHERE (CONDITIONS); 例子：
考虑一个表名 STUDENT
| S_NO. | s_NAME | S_AGE | S_SECTION |
|---|---|---|---|
| 1 | Yash | 20 | A |
| 2 | Vishwash | 21 | A |
| 3 | Vishesh | 19 | B |
| 4 | Shivam | 23 | A |
| 5 | Vasu | 21 | B |
| 6 | Shrey | 20 | C |
问题：
我们必须选择那些年龄小于 22 且部分为 A 的 STUDENT 表实例。
询问：
SELECT *
FROM STUDENT
WHERE S_AGE<22 AND S_SECTION='A'; 输出：
这里的 FROM 子句选择应该应用 WHERE 子句的表，WHERE 子句检查这两个条件以找到数据集的哪些实例满足它们。
| S_NO. | s_NAME | S_AGE | S_SECTION |
|---|---|---|---|
| 1 | Yash | 20 | A |
| 2 | Vishwash | 21 | A |
FROM 子句和 WHERE 子句的区别：
| S_NO. | FROM Clause | WHERE Clause |
|---|---|---|
| 1. | It is used to select the dataset on which manipulation has to be done. | It is used for checking some conditions to filter result |
| 2. | We provide some dataset into the FROM clause as a input. | In WHERE clause we give some condition as input. |
| 3. | FROM clause selects dataset to provide it to WHERE clause for applying conditions given in query. | WHERE clause act as selector which filters required instances from dataset provide by FROM clause. |
| 4. | FROM clause is mandatory because if there is no dataset, no manipulation can be performed. | WHERE is optional, we use it only in case of condition checking. |