How to remove duplicate data in oracle
In Oracle, you can use the Distinct keyword to remove duplicate data and filter duplicate rows in the result set. This keyword can ensure that the value of the specified column or columns returned in the SELECT clause is unique. ;The syntax is "SELECT DISTINCT field name FROM data table name;".
The operating environment of this tutorial: Windows 7 system, Oracle 11g version, Dell G3 computer.
In Oracle, you can use the Distinct keyword to remove duplicate data.
Oracle Distinct keyword
Use the DISTINCT clause in the SELECT statement to filter duplicate rows in the result set. It ensures that the values returned in the specified column or columns in the SELECT clause are unique.
The following explains the syntax of the SELECT DISTINCT statement:
SELECT DISTINCT column_name FROM table_name;
In the above syntax, table_namethe column_name# of the table ## Values in columns will be compared to filter out duplicates.
SELECT clause, as follows:
SELECT DISTINCT column_1, column_2, ... FROM table_name;In this syntax, ## The combination of values in #column_1
, column_2, and column_n is used to determine the uniqueness of the data. The
clause can only be used in the SELECT statement. Please note that
is not a synonym for the SQL standard UNIQUE. It is a good habit to always use DISTINCT instead of UNIQUE. Oracle DISTINCT Example
Here are some examples of how to use
SELECT DISTINCT to see how it works. 1. Example of Oracle DISTINCT applied on one column
View the Contacts (
contacts) table in the sample database:
The following example retrieves the names of all contacts:
SELECT first_name FROM contacts ORDER BY first_name;
Execute the above query statement and get the following results-
The query returned
row means that the contact (contacts) table has 319 rows. To obtain unique contact names, you can add the
keyword to the SELECT statement above, as follows: <pre class="brush:php;toolbar:false">SELECT DISTINCT first_name
FROM contacts
ORDER BY first_name;</pre> Execute the above query statement and get the following results -
The query returned
rows, indicating the contact (contacts) table There are 17 rows that are duplicates and they have been filtered. 2. Oracle DISTINCT application multi-column example
Look at the
order_items table below. The structure of the table is as follows:
The following statement selects different product IDs and quantities from the
table: <pre class="brush:php;toolbar:false">SELECT
DISTINCT product_id,
quantity
FROM
ORDER_ITEMS
ORDER BY product_id;</pre>Execute the above query statement and get the following results-
In this example, the values of the
and quantity columns are both used to evaluate the uniqueness of the rows in the result set. 3. Oracle DISTINCT and NULL
treats NULL values as duplicate values. If you use the SELECT DISTINCT statement to query data from a column with multiple NULL values, the result set contains only one NULL value. Please refer to the
table in the sample database, the structure is as follows -
The following statements are from
Retrieve data with multiple NULL values in the column: <pre class="brush:php;toolbar:false">SELECT DISTINCT state
FROM locations
ORDER BY state NULLS FIRST;</pre>Execute the above example code and get the following results-
value is returned. Recommended tutorial: "
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