MySQL 自学笔记之：数据库查询 DQL（Data Query Lanuage）基本查询

> DQL (Data Query Lanuage) 基本查询

基于查询语句，由select 语句及其子语句组成。

SELECT: 基本语句

FROM: 必备子语句

WHERE: 可选子语句

ORDER BY: 可选子语句

1. 主要（典型）语法syntax:

SELECT [* | ALL | DISTINCT column1， colum2 ]

FROM TABLE1 [, TABLE2]

WHERE [condition1 | expression1 and|or condition 2 | expression2]

ORDER BY colum1 | integer [ASC | DESC]

2. 统计表中的记录数量：

SELECT COUNT(*）

FROM TABLE_NAME;

3. 使用字段别名：

SELECT column_name alias_name

FROM TABLE_NAME;

选项*: 表示输出表中的全部字段

选项ALL: 默认的操作

选项DISTINCT： 用于禁止输出重复的行

选项ORDER BY：默认为升序排列。ASC: 升序，DESC: 降序

注意：

不同的数据库系统，对大小写的敏感性不同。

mysql 不区分大小写，而Oracle 区分大小写。

练习：

For exmaple:

products_tbl，是我之前建好的表。可用用desc查看其字段，用select * from products_tbl 读取所有数据。

实例操作如下：

mysql> desc products_tbl;
+----------+--------------+------+-----+---------+-------+
| Field    | Type         | Null | Key | Default | Extra |
+----------+--------------+------+-----+---------+-------+
| pro_id   | varchar(10)  | NO   |     | NULL    |       |
| pro_desc | varchar(20)  | NO   |     | NULL    |       |
| cost     | decimal(6,2) | NO   |     | NULL    |       |
+----------+--------------+------+-----+---------+-------+
3 rows in set (0.01 sec)

mysql> SELECT * FROM products_tbl;
+--------+-----------------+-------+
| pro_id | pro_desc        | cost  |
+--------+-----------------+-------+
| 7725   | Leather gloves  | 24.99 |
| 7723   | trousers        | 15.99 |
| 303    | kiddie grab bag |  4.99 |
| 7710   | trousers        | 14.66 |
| 7710   | trousers        | 19.66 |
+--------+-----------------+-------+
5 rows in set (0.00 sec)

mysql> SELECT pro_id
-> FROM products_tbl;
+--------+
| pro_id |
+--------+
| 7725   |
| 7723   |
| 303    |
| 7710   |
| 7710   |
+--------+
5 rows in set (0.00 sec)

mysql> SELECT pro_id, pro_desc
-> FROM products_tbl;
+--------+-----------------+
| pro_id | pro_desc        |
+--------+-----------------+
| 7725   | Leather gloves  |
| 7723   | trousers        |
| 303    | kiddie grab bag |
| 7710   | trousers        |
| 7710   | trousers        |
+--------+-----------------+
5 rows in set (0.00 sec)

mysql> SELECT *
-> FROM products_tbl
-> WHERE cost < 10 or cost > 20;
+--------+-----------------+-------+
| pro_id | pro_desc        | cost  |
+--------+-----------------+-------+
| 7725   | Leather gloves  | 24.99 |
| 303    | kiddie grab bag |  4.99 |
+--------+-----------------+-------+
2 rows in set (0.00 sec)

mysql> SELECT *
-> FROM products_tbl
-> order by cost ;
+--------+-----------------+-------+
| pro_id | pro_desc        | cost  |
+--------+-----------------+-------+
| 303    | kiddie grab bag |  4.99 |
| 7710   | trousers        | 14.66 |
| 7723   | trousers        | 15.99 |
| 7710   | trousers        | 19.66 |
| 7725   | Leather gloves  | 24.99 |
+--------+-----------------+-------+
5 rows in set (0.00 sec)

mysql> SELECT *
-> FROM products_tbl
-> order by cost  ASC;
+--------+-----------------+-------+
| pro_id | pro_desc        | cost  |
+--------+-----------------+-------+
| 303    | kiddie grab bag |  4.99 |
| 7710   | trousers        | 14.66 |
| 7723   | trousers        | 15.99 |
| 7710   | trousers        | 19.66 |
| 7725   | Leather gloves  | 24.99 |
+--------+-----------------+-------+
5 rows in set (0.00 sec)

mysql> SELECT *
-> FROM products_tbl
-> order by cost  DESC;
+--------+-----------------+-------+
| pro_id | pro_desc        | cost  |
+--------+-----------------+-------+
| 7725   | Leather gloves  | 24.99 |
| 7710   | trousers        | 19.66 |
| 7723   | trousers        | 15.99 |
| 7710   | trousers        | 14.66 |
| 303    | kiddie grab bag |  4.99 |
+--------+-----------------+-------+
5 rows in set (0.00 sec)

mysql> SELECT count(*)
-> FROM products_tbl;
+----------+
| count(*) |
+----------+
|        5 |
+----------+
1 row in set (0.00 sec)

mysql> SELECT COUNT(*)
-> FROM products_tbl
-> where cost < 20;
+----------+
| COUNT(*) |
+----------+
|        4 |
+----------+
1 row in set (0.00 sec)

mysql> SELECT pro_id  id
-> FROM products_tbl;
+------+
| id   |
+------+
| 7725 |
| 7723 |
| 303  |
| 7710 |
| 7710 |
+------+
5 rows in set (0.00 sec)

mysql> SELECT pro_id, pro_id id
-> FROM products_tbl;
+--------+------+
| pro_id | id   |
+--------+------+
| 7725   | 7725 |
| 7723   | 7723 |
| 303    | 303  |
| 7710   | 7710 |
| 7710   | 7710 |
+--------+------+
5 rows in set (0.00 sec)

mysql> SELECT PRO_ID, PRO_ID ID, PRO_DESC, PRO_DESC PRODUCT
-> FROM PRODUCTS_TBL;
+--------+------+-----------------+-----------------+
| PRO_ID | ID   | PRO_DESC        | PRODUCT         |
+--------+------+-----------------+-----------------+
| 7725   | 7725 | Leather gloves  | Leather gloves  |
| 7723   | 7723 | trousers        | trousers        |
| 303    | 303  | kiddie grab bag | kiddie grab bag |
| 7710   | 7710 | trousers        | trousers        |
| 7710   | 7710 | trousers        | trousers        |
+--------+------+-----------------+-----------------+
5 rows in set (0.00 sec)

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