SQL-SELECT
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This article talks about the SELECT statement about the establishment of the database table structure, I use postgreSQL to achieve it, the others should be similar, mainly to understand SQL statements
Select statement is a statement used to perform query operations. Simple to say is actually simple, but difficult to say. The select statement can be very simple to query all the data in a table like this.
Of course, you can also write very complex query statements, as shown below (I picked up a picture on the Internet and brought it over):
Therefore, select is the core of the query, and it is necessary for us to study it.
grammar
The grammar part is always so simple
SELECT column_name [, column_name]
FROM table1 [, table2]
WHERE [CONDITIONS]
The syntax of the SELECT statement is to select the column name first, then specify a certain TABLE from FROM, and finally you can choose to add some additional conditions.
After understanding the basic grammar, the focus is on how to use it. This is the secret of the SELECT statement.
Instance
Still based on our database operations, yes yes
- Query all student information
SELECT * FROM students;
- Query the student ID, name and date of birth of all male students
SELECT sno,sname,sbirthday FROM student WHERE ssex='M';
- Query the student ID, name, gender, date of birth of all female students born before "1980-01-01"
select sno,sname,ssex from student where (ssex='F' and sbirthday < '1980-1-1');
- Query the student ID, name, gender, and date of birth of all male students with the surname "Li"
select sno,sname,ssex,sbirthday from student where sname like 'L%';
