Select
用途：
从指定表中取出指定的列的数据
语法：
SELECT column_name(s) FROM table_name |
解释：
从数据库中选取资料列，并允许从一或多个资料表中，选取一或多个资料列或资料行。SELECT 陈述式的完整语法相当复杂，但主要子句可摘要为：
SELECT select_list
[ INTO new_table ]
FROM table_source
[ WHERE search_condition ]
[ GROUP BY group_by_expression ]
[ HAVING search_condition ]
[ ORDER BY order_expression [ ASC | DESC ] ]
例：
“Persons” 表中的数据有
LastName | FirstName | Address | City |
Hansen | Ola | Timoteivn 10 | Sandnes |
Svendson | Tove | Borgvn 23 | Sandnes |
Pettersen | Kari | Storgt 20 | Stavanger |
选出字段名” LastName”、” FirstName” 的数据
SELECT LastName,FirstName FROM Persons |
返回结果:
LastName | FirstName |
Hansen | Ola |
Svendson | Tove |
Pettersen | Kari |
选出所有字段的数据
SELECT * FROM Persons |
返回结果:
LastName | FirstName | Address | City |
Hansen | Ola | Timoteivn 10 | Sandnes | update语法大全
Svendson | Tove | Borgvn 23 | Sandnes |
Pettersen | Kari | Storgt 20 | Stavanger |
Where
用途：
被用来规定一种选择查询的标准
语法：
SELECT column FROM table WHERE column condition value |
下面的操作符能被使用在WHERE中：
=,<>,>,<,>=,<=,BETWEEN,LIKE
注意： 在某些SQL的版本中不等号< >能被写作为!=
解释：
SELECT语句返回WHERE子句中条件为true的数据
例：
从” Persons”表中选出生活在” Sandnes” 的人
SELECT * FROM Persons WHERE City='Sandnes' |
"Persons" 表中的数据有:
LastName | FirstName | Address | City | Year |
Hansen | Ola | Timoteivn 10 | Sandnes | 1951 |
Svendson | Tove | Borgvn 23 | Sandnes | 1978 |
Svendson | Stale | Kaivn 18 | Sandnes | 1980 |
Pettersen | Kari | Storgt 20 | Stavanger | 1960 |
返回结果:
LastName | FirstName | Address | City | Year |
Hansen | Ola | Timoteivn 10 | Sandnes | 1951 |
Svendson | Tove | Borgvn 23 | Sandnes | 1978 |
Svendson | Stale | Kaivn 18 | Sandnes | 1980 |
And & Or
用途：
在WHERE子句中AND和OR被用来连接两个或者更多的条件
解释：
AND在结合两个布尔表达式时，只有在两个表达式都为 TRUE 时才传回 TRUE
OR在结合两个布尔表达式时，只要其中一个条件为 TRUE 时，OR便传回 TRUE
例：
"Persons" 表中的原始数据:
LastName | FirstName | Address | City |
Hansen | Ola | Timoteivn 10 | Sandnes |
Svendson | Tove | Borgvn 23 | Sandnes |
Svendson | Stephen | Kaivn 18 | Sandnes |
用AND运算子来查"Persons" 表中FirstName为”Tove”而且LastName为” Svendson”的数据
SELECT * FROM Persons WHERE FirstName='Tove' AND LastName='Svendson' |
返回结果:
LastName | FirstName | Address | City |
Svendson | Tove | Borgvn 23 | Sandnes |
用OR运算子来查"Persons" 表中FirstName为”Tove”或者LastName为” Svendson”的数据
SELECT * FROM Persons WHERE firstname='Tove' OR lastname='Svendson' |
返回结果:
LastName | FirstName | Address | City |
Svendson | Tove | Borgvn 23 | Sandnes |
Svendson | Stephen | Kaivn 18 | Sandnes |
你也能结合AND和OR (使用括号形成复杂的表达式),如:
SELECT * FROM Persons WHERE (FirstName='Tove' OR FirstName='Stephen') AND LastName='Svendson' |
返回结果:
LastName | FirstName | Address | City |
Svendson | Tove | Borgvn 23 | Sandnes |
Svendson | Stephen | Kaivn 18 | Sandnes |
Between…And
用途：
指定需返回数据的范围
语法：
SELECT column_name FROM table_name WHERE column_name BETWEEN value1 AND value2 |
例：
“Persons”表中的原始数据
LastName | FirstName | Address | City |
Hansen | Ola | Timoteivn 10 | Sandnes |
Nordmann | Anna | Neset 18 | Sandnes |
Pettersen | Kari | Storgt 20 | Stavanger |
Svendson | Tove | Borgvn 23 | Sandnes |
用BETWEEN…AND返回LastName为从”Hansen”到”Pettersen”的数据：
SELECT * FROM Persons WHERE LastName BETWEEN 'Hansen' AND 'Pettersen' |
返回结果:
LastName | FirstName | Address | City |
Hansen | Ola | Timoteivn 10 | Sandnes |
Nordmann | Anna | Neset 18 | Sandnes |
Pettersen | Kari | Storgt 20 | Stavanger |
为了显示指定范围之外的数据，也可以用NOT操作符：
SELECT * FROM Persons WHERE LastName NOT BETWEEN 'Hansen' AND 'Pettersen' |
返回结果:
LastName | FirstName | Address | City |
Svendson | Tove | Borgvn 23 | Sandnes |
Distinct
用途：
DISTINCT关键字被用作返回唯一的值
语法：
SELECT DISTINCT column-name(s) FROM table-name |
解释：
当column-name(s)中存在重复的值时，返回结果仅留下一个
例：
“Orders”表中的原始数据
Company | OrderNumber |
Sega | 3412 |
W3Schools | 2312 |
Trio | 4678 |
W3Schools | 6798 |
用DISTINCT关键字返回Company字段中唯一的值：
SELECT DISTINCT Company FROM Orders |
返回结果:
Company |
Sega |
W3Schools |
Trio |
Order by
用途：
指定结果集的排序
语法：
SELECT column-name(s) FROM table-name ORDER BY { order_by_expression [ ASC | DESC ] } |
解释：
指定结果集的排序，可以按照ASC(递增方式排序，从最低值到最高值)或者DESC(递减方式排序，从最高值到最低值)的方式进行排序，默认的方式是ASC
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