sql学习之1-create、select

1、mysql创建表

CREATE TABLE test_tab (
id INT NOT NULL,
name VARCHAR(10),
age INT,
PRIMARY KEY (id)
);

2、sqlserver/oracle创建表

CREATE TABLE test_tab (
id INT NOT NULL PRIMARY KEY,
name VARCHAR(10),
age INT
);

 3、除去重复数据DISTINCT

SELECT DISTINCT 列名称 FROM 表名称
如下数据：

Company	OrderNumber
IBM	3532
W3School	2356
Apple	4698
W3School	6953

SELECT Company FROM Orders

结果如下：

Company

IBM

W3School

Apple

W3School

SELECT DISTINCT Company FROM Orders 

结果如下：

Company

IBM

W3School

Apple

4、对查询结果集的列进行重命名AS，连接多个查询UNION

SELECT 
    'bptest',
     COUNT(*)
FROM
    bptest
UNION 
SELECT 
    'test_tab',
     COUNT(*)
FROM
    test_tab;

查询结果：

+----------+----------+
| bptest | COUNT(*) |
+----------+----------+
| bptest | 2 |
| test_tab | 10000000 |
+----------+----------+

修改后：

SELECT 
    'bptest' AS table_name,
     COUNT(*) AS table_row
FROM
    bptest
UNION 
SELECT 
    'test_tab' AS table_name,
     COUNT(*) AS table_row
FROM
    test_tab;

查询结果：

+------------+-----------+
| table_name | table_row |
+------------+-----------+
| bptest | 2 |
| test_tab | 10000000 |
+------------+-----------+