Writing Oracle SQL statements requires following the following steps: 1. Connect to the database; 2. Select the columns to be retrieved; 3. Specify conditions (optional); 4. Sort the results (optional); 5 . Limit the number of results (optional); 6. Use aggregate functions (optional); 7. Use subqueries (optional); 8. Use joins (optional).
How to write SQL statements in Oracle
Oracle SQL (Structured Query Language) is a powerful Tools for retrieving, manipulating, and updating data from Oracle databases. To write a valid SQL statement, you need to follow these steps:
1. Connect to the database
CONNECT username/password@database_name;
2. Select the columns to retrieve
SELECT 列名 FROM 表名;
3. Specify conditions (optional)
Use the WHERE clause to filter the query:
SELECT 列名 FROM 表名 WHERE 条件;
4. Filter the results Sort (optional)
Use the ORDER BY clause to sort the results:
SELECT 列名 FROM 表名 ORDER BY 列名 ASC/DESC;
5. Limit the number of results (optional)
Use LIMIT clause to limit the number of retrieved results:
SELECT 列名 FROM 表名 LIMIT 行数;
6. Use aggregate functions (optional)
Aggregation functions (such as SUM, COUNT, AVG ) can perform calculations on data:
SELECT 聚合函数(列名) FROM 表名 GROUP BY 分组列;
7. Using subqueries (optional)
Subqueries are queries nested in the main query and are used for retrieval Intermediate results for the main query:
SELECT 列名 FROM 表名 WHERE 条件 IN (SELECT 列名 FROM 子查询);
8. Using joins (optional)
Connections are used to retrieve data from multiple tables:
SELECT 列名 FROM 表名1 JOIN 表名2 ON 连接条件;
Example:
Here's how to retrieve the names and salaries of all employees from the "EMP" table:
SELECT ename, sal FROM emp;
Here's how to sort the results in descending order of salary Sorting:
SELECT ename, sal FROM emp ORDER BY sal DESC;
Here's how to use a subquery to find employees with above-average salaries:
SELECT ename FROM emp WHERE sal > (SELECT AVG(sal) FROM emp);
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