How to write oracle query statement

Oracle is a commonly used relational database management system, and its query statements are very flexible and powerful. This article will introduce in detail how to write Oracle query statements and their common syntax.

Basic syntax

Oracle’s query statements mainly include six keywords: SELECT, FROM, WHERE, GROUP BY, HAVING and ORDER BY. Below we introduce their functions and usage methods one by one.

1. SELECT

The SELECT statement is used to select a set of data from one or more tables. Its format is as follows:

SELECT [column1, column2 , ...] FROM [table1, table2, ...];

Among them, [column1, column2, ...] represents the columns to be queried, and the wildcard character (*) can be used to query all columns; [table1, table2, ...] represents the table to be queried, and aliases can be used to simplify the table name.

2. FROM

The FROM statement is used to specify the table to be queried in the SELECT statement. Its format is as follows:

SELECT [column1, column2, .. .] FROM [table1, table2, ...];

Where [table1, table2, ...] represents the table to be queried, and aliases can be used to simplify the table name.

3. WHERE

The WHERE statement is used to specify query conditions. Its format is as follows:

SELECT [column1, column2, ...] FROM [table1 , table2, ...] WHERE [condition];

Where [condition] represents the query condition, you can use comparison operators (=, >, <, >=, <=, < ;>), logical operators (AND, OR, NOT), wildcards (LIKE), and keywords such as IN and BETWEEN to combine query conditions.

4. GROUP BY

The GROUP BY statement is used to group query results by specified columns. Its format is as follows:

SELECT [column1, column2, ...] FROM [table1, table2, ...] WHERE [condition] GROUP BY [column];

where [column] indicates which column to group by.

5. HAVING

The HAVING statement is used to further filter the query results grouped by GROUP BY. Its format is as follows:

SELECT [column1, column2 , ...] FROM [table1, table2, ...] WHERE [condition] GROUP BY [column] HAVING [condition];

where [condition] represents further filtering conditions, and comparison operations can be used operators (=, >, <, >=, <=, <>), logical operators (AND, OR, NOT), wildcards (LIKE), and keywords such as IN and BETWEEN to combine query conditions .

6. ORDER BY

The ORDER BY statement is used to sort query results. Its format is as follows:

SELECT [column1, column2, ... ] FROM [table1, table2, ...] WHERE [condition] GROUP BY [column] HAVING [condition] ORDER BY [column];

where [column] indicates which column to sort by, you can use ASC (ascending order) and DESC (descending order) are two keywords to specify the sort order.

Extended syntax

In addition to the basic syntax, Oracle also has some extended syntax that can implement queries more flexibly.

1. Commonly used functions

Oracle supports a variety of commonly used functions, such as mathematical functions (SQRT, ROUND, TRUNC), character functions (SUBSTR, LOWER, UPPER, LENGTH), Date functions (TO_DATE, TO_CHAR, ADD_MONTHS), aggregate functions (SUM, AVG, MAX, MIN, COUNT), etc. These functions can be used to perform custom calculations or format output on query results.

2. Subquery

A subquery can nest one or more query statements in the main query, and the returned result will be used as one of the conditions of the main query. For example:

SELECT [column1, column2, ...] FROM [table1, table2, ...] WHERE [column] IN (SELECT [column] FROM [table] WHERE [condition]);

Among them, the subquery is used to filter out the records that meet the conditions and use them as the conditions of the main query.

3. JOIN

The JOIN statement is used to join two or more tables. Its format is as follows:

SELECT [column1, column2, ... ] FROM [table1] JOIN [table2] ON [condition];

Where [condition] represents the connection condition, you can use comparison operators (=, >, <, >=, <= , <>), logical operators (AND, OR, NOT), wildcards (LIKE), and keywords such as IN and BETWEEN to combine connection conditions.

Summary

Oracle query statements are very flexible and powerful and can meet query operations with different needs. This article introduces the basic syntax of Oracle query statements and some commonly used extended syntax. I hope it will be helpful for everyone to learn and use Oracle database.

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