How to query row data in oracle

Oracle is a very popular relational database management system, which contains a rich query language that can retrieve, filter, sort and other operations on data in the database. Oracle provides a variety of methods for querying row data. Here are some commonly used query methods.

1. Use the SELECT statement to query row data

The SELECT statement is the most basic and commonly used query statement in Oracle. It can query row data under specified conditions. For example, to query employee information in the employee table whose salary is greater than 5000, the SQL statement is as follows:

SELECT * FROM employee WHERE salary > 5000;

Among them, * represents all columns, use this symbol Data of all columns can be queried. employee is the table name to be queried, salary is the column name to be queried, > is the comparison operator, and 5000 is the data to be queried. After running the above statement, Oracle will return all row data that meets the conditions.

2. Use the DISTINCT keyword to query unique data

Sometimes, we need to query the unique value of a column. At this time, we need to use the DISTINCT keyword. For example, to query the number of employees with different professional titles in the employee table, the SQL statement is as follows:

SELECT COUNT(DISTINCT title) FROM employee;

Among them, the COUNT function is used to calculate the number of rows, and the DISTINCT keyword Used for deduplication, title is the column name to be queried.

3. Use the ORDER BY clause to sort row data

The ORDER BY clause can sort the query results according to the specified column. For example, to query all employee information in the employee table, sorted by joining time from morning to night, the SQL statement is as follows:

SELECT * FROM employee ORDER BY hire_date ASC;

where ASC means ascending order , use DESC to sort in descending order. After running the above statement, Oracle will sort the query results according to the value of the hire_date column and return them.

4. Use the LIMIT statement to limit query results

When querying data, sometimes we only need to query the first few pieces of data. In this case, we need to use the LIMIT statement. The LIMIT statement in Oracle is similar to the LIMIT statement of MySQL and can limit the number of query results. For example, to query the first 10 employee information in the employee table, the SQL statement is as follows:

SELECT * FROM employee WHERE rownum <= 10;

Among them, rownum is Oracle's built-in pseudo column, which means Rows. Setting rownum to less than or equal to 10 can limit the number of query results.

The above are the more commonly used methods for querying row data in Oracle. According to specific business needs, we can flexibly use the above methods to query data. At the same time, when performing data query, we also need to pay attention to avoid problems such as slow query or server downtime caused by querying too much data.

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