Java Fundamentals (chapter IX)

Java Fundamentals (chapter IX)

I. Understanding the mechanism of the query

The client application (c/S, b/s) sends a SELECT statement to the DB of the backend server to query, returning the result set (virtual table) to the client application

Second, Select Statement

1. Querying all columns and rows in the table

SELECT * FROM table name

2. Query data according to conditions

SELECT * FROM table name where condition

3, query the data of some columns

Select Column Name 1, column Name 2 .... from table name [where condition]

4. Alias for column name

Way 1:select column name as alias from table name [where condition]

Method 2:select column name alias from table name [where condition]

Way 3:select alias = column name from table name [where condition]

5. Constant column

Select ' constant value ' as constant column name from table name [where condition]

6. Fetch the number of rows in the table

Method 1: Extract the fixed line

Select top row sequence name from table name [where condition]

Method 2: Extract data rows by percentage

Select top Percent Percent column name from table name [where condition]

7, query data, and sort

Ascending:

Select column Names list from table name [where Condition]torder by sort column name ' ASC '

Descending:

Select column list from table name [where condition]

Order by sort column name desc

8. Querying a table for a column with a null value in the data row

Select list List from table name where column name = ' or column name is null

Java Fundamentals (chapter IX)