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How to calculate percentages in sql

There are three main ways to calculate percentages using SQL: direct calculation, use window functions, and use CASE expressions. Direct calculations are obtained by dividing the numerator by the denominator and multiplying by 100; window functions allow percentages to be calculated based on the group by the denominator value; while CASE expressions can calculate different percentages based on conditions.

Apr 10, 2025 am 10:06 AM

How to convert timestamps to date format using sql

To convert timestamps in SQL to date format, you can use the TO_CHAR() function: TO_CHAR(timestamp_column, 'YYYY-MM-DD') to convert timestamps to date strings. Other date formats can be used, such as 'YYY-MM-DD HH24:MI:SS' (date and time) and 'YYY-MM-DD HH:MI:SS AM' (date and time, with AM/PM).

Apr 10, 2025 am 10:03 AM

How to add a column to a table in sql

How to add a column to a SQL table? Use the ALTER TABLE statement to specify the table name, column name, and data type; optionally set NOT NULL constraints and default values; run the ALTER TABLE statement to add a new column to the table.

Apr 10, 2025 am 10:00 AM

How to disconnect database usage of SQL

You can use SQL statements to disconnect database connections, including: 1. CLOSE statement: close the specified connection immediately; 2. DISCONNECT statement: close the connection and release the resources; 3. END TRANSACTION statement: submit/roll back the transaction and disconnect; 4. EXIT statement: terminate the script and close all connections; 5. Connection object method (specific to the database system). Additionally, it is recommended to commit or roll back unfinished transactions before disconnecting.

Apr 10, 2025 am 09:57 AM

python

How to clean specific data in SQL

How to clean up specific data using SQL: Determine the data to clean. Use the DELETE statement to delete data according to conditional conditions. Use the TRUNCATE statement to quickly delete all data in the table (use with caution). Optimize queries to improve efficiency. Backup data before cleaning.

Apr 10, 2025 am 09:54 AM

How to write loop statements in SQL

There are no traditional loop statements in SQL, but you can simulate loops using recursive CTE: create a recursive CTE that contains a "iteration" column to track the number of loops. Recursive query updates the "iteration" column and satisfies the loop condition. Use the SELECT statement to select the required data from the CTE.

Apr 10, 2025 am 09:51 AM

How to filter duplicates in sql

Methods for filtering duplicate records in SQL include: using the DISTINCT keyword to remove duplicate values. Use the GROUP BY clause to group the results and return the first row of each group. Use the COUNT() function to find the values that appear multiple times and filter them further. Use a temporary table to save unique values and retrieve records from them. Use the window function to assign a unique line number to each line and select the first non-repeat line.

Apr 10, 2025 am 09:48 AM

How to implement column to row in sql

There are two ways to implement column-to-line conversion operations in SQL: use the UNION operator to realize column-to-line conversion by vertically merging the query results. Use the PIVOT function to convert column data into row data, where the syntax of the PIVOT function is: PIVOT ( aggregate_function(column_name) FOR pivot_column_name IN (value1, value2, ...) ) FROM table_name

Apr 10, 2025 am 09:45 AM

聚合函数

MongoDB: Security, Performance, and Stability

MongoDB excels in security, performance and stability. 1) Security is achieved through authentication, authorization, data encryption and network security. 2) Performance optimization depends on indexing, query optimization and hardware configuration. 3) Stability is guaranteed through data persistence, replication sets and sharding.

Apr 10, 2025 am 09:43 AM

MongoDB数据库性能

How to round up sql function

The CEILING() function in SQL rounds upwards and rounds the numeric value to the nearest integer. The syntax is CEILING(numeric_expression). Example: Round upwards 2.5 to 3, and -10.2 to -10. CEILING() is suitable for numerical data types. The NULL value returns NULL, rounding the first digit after the decimal point.

Apr 10, 2025 am 09:42 AM

phpMyAdmin: Simplifying SQL Queries and Operations

phpMyAdmin simplifies SQL workflow, performs queries and manages databases through graphical interfaces. 1) Create, modify and delete databases and tables; 2) Execute SQL queries; 3) Import and export data; 4) Manage user permissions.

Apr 10, 2025 am 09:41 AM

SQL

Oracle Backup & Recovery: Ensuring Data Integrity & Availability

The core purpose of Oracle backup and recovery is to quickly restore the database to a consistent state when data is lost or corrupted. 1. Backup phase: Copy the database file to the backup media through RMAN or other tools. 2. Recovery phase: When a database fails, first restore the backup data, and then restore to the pre-failed state by applying the redo log file.

Apr 10, 2025 am 09:40 AM

数据恢复Oracle备份

How to query field duplicate data in SQL

Repeated field data can be queried using SQL's GROUP BY and COUNT() functions, DISTINCT and COUNT() functions, or window function ROW_NUMBER().

Apr 10, 2025 am 09:39 AM

How to identify and optimize slow queries in MySQL? (slow query log, performance_schema)

To optimize MySQL slow query, slowquerylog and performance_schema need to be used: 1. Enable slowquerylog and set thresholds to record slow query; 2. Use performance_schema to analyze query execution details, find out performance bottlenecks and optimize.

Apr 10, 2025 am 09:36 AM

性能优化MySQL慢查询