How to query column data in oracle

Oracle is a relational database management system whose most important function is to store and retrieve data. Good querying skills are key when working with databases, as tens of millions of rows of data need to be sifted out to find the information you need. This article will introduce how to query column data in Oracle database.

1. SELECT statement

The SELECT statement is the most commonly used query statement in Oracle. Its basic format is as follows:

SELECT column1, column2, ...
FROM table_name;

Among them, "column1, column2, ..." is the column you want to retrieve, "table_name" is the table you want to retrieve.

For example, if you want to retrieve the name and age columns of the students table, you can use the following statement:

SELECT name, age
FROM students;

2. WHERE Clause

If you only want to retrieve specific rows that match certain conditions, you need to use the WHERE clause. The format of this clause is as follows:

SELECT column1, column2, ...
FROM table_name
WHERE condition;

Among them, "condition" is the condition you want to check.

For example, if you want to retrieve students who are 18 years or older in the students table, you can use the following statement:

SELECT name, age
FROM students
WHERE age > = 18;

3. LIKE operator

The LIKE operator is used for fuzzy search to find data containing a certain string. The following is the format of the LIKE operator:

SELECT column1, column2, ...
FROM table_name
WHERE columnX LIKE pattern;

where "pattern" is what you want to find string pattern.

For example, if you need to find students whose names contain "Zhang" in the students table, you can use the following statement:

SELECT name, age
FROM students
WHERE name LIKE ' %张%';

In this example, "%" is a wildcard character, representing any character, including 0 characters and multiple characters.

4. ORDER BY clause

The ORDER BY clause is used to sort the result set in ascending or descending order. You can use the following format:

SELECT column1, column2, ...
FROM table_name
ORDER BY columnX;

where "columnX" is the column you want to sort. By default, data is sorted in ascending order.

For example, if you want to view student information sorted by age in the students table, you can use the following statement:

SELECT name, age
FROM students
ORDER BY age;

5. GROUP BY clause

The GROUP BY clause is used to group query results based on one or more columns. The basic format of this statement is as follows:

SELECT column1, column2, ..., aggregate_function(columnY)
FROM table_name
GROUP BY columnZ;

where "columnY" is The column you want to aggregate, "columnZ" is the column you want to group by.

For example, if you want to see the number of students of different age groups, you can use the following statement:

SELECT age, COUNT(*)
FROM students
GROUP BY age;

In this example, use the COUNT(*) function to count the number of students in each age group.

6. HAVING clause

The HAVING clause filters the group results after the GROUP BY clause. The usage is as follows:

SELECT column1, column2, ..., aggregate_function(columnY)
FROM table_name
GROUP BY columnZ
HAVING condition;

where, "condition" is the condition you want to apply to the grouped results.

For example, if you only want to view the number of students who are 18 years or older, you can use the following statement:

SELECT age, COUNT(*)
FROM students
GROUP BY age
HAVING age >= 18;

7. JOIN statement

The JOIN statement is used to establish a relationship between two or more tables and merge their data into a result set. The following is the format of INNER JOIN:

SELECT table1.column1, table2.column2, ...
FROM table1
INNER JOIN table2
ON table1.columnX = table2.columnX;

Among them, "columnX" is the column you want to match.

For example, if you want to retrieve detailed information about the courses selected by students in the students table and courses table, you can use the following statement:

SELECT students.name, courses.course_name
FROM students
INNER JOIN courses
ON students.course_id = courses.course_id;

This query uses the course_id column between the student table and the course table to establish a connection.

Conclusion

By using the above SELECT statement, WHERE clause, LIKE operator, ORDER BY clause, GROUP BY clause, HAVING clause and JOIN statement, you can Easily query column data. Query skills are key to becoming a successful data manager, and as you continue to practice and practice, you will become more and more proficient.

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