DML(数据操作语言)

数据操作语言 (DML，Data Manipulation Language )

SQL SELECT

The SELECT statement is used to select data from a database.

从数据库表中获取数据。

语法

# 查询表中具体字段
SELECT column_name,column_name FROM table_name;

或

# 全表查询
SELECT * FROM table_name;

SQL SELECT DISTINCT

The SELECT DISTINCT statement is used to return only distinct (different) values.

关键词 DISTINCT 用于返回唯一不同的值。

语法

SELECT DISTINCT column_name,column_name FROM table_name;

SQL UPDATE

The UPDATE statement is used to update existing records in a table.

Update 语句用于修改表中现有的数据。

语法

UPDATE table_name
SET column1=value1,column2=value2,...
WHERE some_column=some_value;

SQL DELETE

The DELETE statement is used to delete records in a table.

DELETE 语句用于删除表中的行。

语法

DELETE FROM table_name
WHERE some_column=some_value;

SQL INSERT INTO

The INSERT INTO statement is used to insert new records in a table.

INSERT INTO 语句用于向表格中插入新的行。

语法

INSERT INTO table_name
VALUES (value1,value2,value3,...);

或

INSERT INTO table_name (column1,column2,column3,...)
VALUES (value1,value2,value3,...);

SQL WHERE Clause

The WHERE clause is used to extract only those records that fulfill a specified criterion.

如需有条件地从表中选取数据，可将 WHERE 子句添加到 SELECT 语句。

语法

SELECT column_name,column_name
FROM table_name
WHERE column_name operator value;

SQL ORDER BY Keyword

The ORDER BY keyword is used to sort the result-set by one or more columns.

The ORDER BY keyword sorts the records in ascending order by default. To sort the records in a descending order, you can use the DESC keyword.

ORDER BY 语句用于根据指定的列对结果集进行排序。

ORDER BY 语句默认按照升序对记录进行排序。如果您希望按照降序对记录进行排序，可以使用 DESC 关键字。

语法

SELECT column_name, column_name
FROM table_name
ORDER BY column_name ASC|DESC, column_name ASC|DESC;