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Technical Articles

Database

SQL

Deleting rows based on criteria from multiple SQL tables.

TodeleterowsbasedoncriteriafrommultipleSQLtables,useJOINsinDELETEstatements,handlecascadingdeleteswithforeignkeys,andperformdeletionsacrossmultipletableswithinatransaction.First,whendeletingrowsfromonetablebasedonanother,employaDELETEstatementwithaJO

Jul 11, 2025 am 02:30 AM

How to combine multiple conditions with AND and OR in a SQL query?

In SQL queries, multiple conditions can be combined using AND and OR to accurately filter data. 1. Use AND to indicate that all conditions must be met at the same time, such as looking for users who are older than 30 and whose city is Beijing; 2. Use OR to indicate that only one of the conditions must be met, such as looking for users who are Beijing or Shanghai; 3. Pay attention to priority when using mixed use, AND takes precedence over OR, and use brackets to clarify the logical relationship to avoid ambiguity; 4. Replacing multiple ORs with IN can make the statement more concise and easy to read, such as matching multiple city names. Use these logical operators and brackets reasonably to write accurate and clear query statements.

Jul 11, 2025 am 02:29 AM

Choosing Appropriate SQL Data Types for Columns.

Selecting the right SQL field data type can improve database performance, storage efficiency and maintainability. 1. The numerical type should be selected according to the value range. TINYINT is suitable for status codes, INT is used for regular primary keys, BIGINT is suitable for high concurrency systems, and DECIMAL is used for amount fields with high accuracy requirements. 2. The string type should be used as needed. CHAR(N) is suitable for fixed-length content, VARCHAR(N) is suitable for variable-length text, and TEXT class is suitable for large paragraphs of text to avoid abuse of VARCHAR(255). 3. DATE is stored in the time type, DATETIME is stored in the date and time. TIMESTAMP is suitable for cross-time zone deployment and automatic conversion of time zones. 4. It is recommended to use TINYINT for enumeration and boolean values.

Jul 11, 2025 am 02:26 AM

Comparing TRUNCATE TABLE and DELETE FROM in SQL.

TRUNCATETABLE is faster and consumes less resources, which is suitable for quickly clearing big data tables; DELETEFROM deletes row by row and supports trigger and conditional control, which is suitable for scenarios where rollback or triggering operations are required; TRUNCATE requires higher permissions and is subject to constraints such as foreign keys, while DELETE is more flexible; TRUNCATE will reset the self-increment counter, while DELETE will retain the original count. 1. TRUNCATE has higher performance because it releases data pages at one time, has few logs and does not trigger triggers; 2. DELETE can roll back in transactions and support conditional filtering, TRUNCATE can also roll back but the behavior is more thorough; 3. TRUNCATE is limited by foreign keys, replication and index views, and requires ALTERTA.

Jul 11, 2025 am 02:21 AM

How to create empty tables with the same structure as another table?

You can use SQL's CREATETABLE statement and SELECT clause to create a table with the same structure as another table. The specific steps are as follows: 1. Create an empty table using CREATETABLEnew_tableASSELECT*FROMexisting_tableWHERE1=0;. 2. Manually add indexes, foreign keys, triggers, etc. when necessary to ensure that the new table is intact and consistent with the original table structure.

Jul 11, 2025 am 01:51 AM

sql Database table structure

SQL basics tutorial for beginners

The key to learning SQL is to master the core commands. First, use SELECT to query data, such as SELECTname, emailFROMcustomersWHEREstate='CA' to retrieve data of specific conditions; second, learn to filter and sort, and implement it through the combination of WHERE, ORDERBY and LIMIT, such as SELECTproduct_name, priceFROMproductsWHEREprice>100ORDERBYpriceDESCLIMIT10; then understand table connections, use INNERJOIN or LEFTJOIN to merge data from multiple tables; finally familiar with INSER

Jul 11, 2025 am 01:49 AM

How to select the top N records from a SQL query?

To get the first N records in SQL queries, you can use specific keywords or functions provided by different database systems. 1. In MySQL and PostgreSQL, use the LIMIT clause and usually combine it with ORDERBY to ensure the correct sorting; 2. In SQLServer, use the TOP keyword, and then it can be followed by ORDERBY to obtain the results after a specific sort; 3. In Oracle or scenarios where compatibility is required, it can be implemented through the ROW_NUMBER() function combined with subqueries. This method is more flexible and suitable for partitioning or complex logic. Regardless of the method, make sure to use ORDERBY to clearly define the order of the data, otherwise the results may be unpredictable.

Jul 11, 2025 am 01:47 AM

Working with JSON data types in modern SQL databases.

Handling JSON data types in modern SQL databases can optimize performance by defining JSON columns, querying with specific functions, and creating indexes. First, you can define JSON type columns in the table, such as MySQL's JSON type or Postgres' JSONB; second, after inserting standard JSON format data, you can extract data through operators, such as Postgres uses @> to find the data containing "dev", and MySQL uses JSON_CONTAINS to achieve similar functions; third, to improve query performance, expression index or virtual columns can be created, such as Postgres uses metadata->>'name

Jul 11, 2025 am 01:34 AM

SQL DATEDIFF function for days, months, years

The DATEDIFF function in SQL is used to calculate the difference between two dates, but its behavior varies according to the selected unit (day, month, year). Use DATEDIFF(day, start_date, end_date) to calculate the number of complete 24-hour cycles spanned during two days. Even if the difference is only one second, it will be counted as 1 day as long as the one-day boundary is crossed; for month differences, DATEDIFF(month, date1, date2) counts the number of months crossed, for example, from February to March, regardless of the specific number of days, regardless of whether the corresponding date is true; year differences are the same, DATEDIFF(year, date1, date2) only counts the year boundary.

Jul 11, 2025 am 01:18 AM

How to drop a column in SQL

To delete columns in SQL tables, the most common method is to use the combination of ALTERTABLE and DROPCOLUMN commands, with the syntax as ALTERTABLE table name DROPCOLUMN column name; for example: ALTERTABLEusersDROPCOLUMNemail; but be careful that the data will be permanently deleted. Before the operation, you should confirm the backup or whether the column is indeed no longer needed. Different databases have differences: MySQL can omit the COLUMN keyword; PostgreSQL requires that COLUMN must be included; Oracle supports standard syntax; SQLite does not directly support it, and it needs to be achieved by creating new tables, copying data, and replacing old tables. Before operation, please be careful: make sure there are no other

Jul 11, 2025 am 12:41 AM

Applying Analytical Operations with SQL Window Functions

To calculate the rankings within each group, you can use the RANK(), ROW_NUMBER() or DENSE_RANK() functions, where RANK() allows to rank but skip subsequent rankings, ROW_NUMBER() generates a unique sequence number, and DENSE_RANK() retains consecutive ranks; for example: SELECTdepartment, employee_name, salary, RANK()OVER(PARTITIONBYdepartmentORDERBYsalaryDESC)ASsalary_rankFROMemployees; to do moving average or cumulative summation, AVG() or SUM can be used

Jul 11, 2025 am 12:31 AM

Handling Duplicate Records When Selecting Data in SQL

There are three main methods for handling duplicate records in SQL queries: 1. Use DISTINCT to remove completely duplicate rows, which is suitable for duplicate situations after multiple fields combinations, and you can find duplicate content through GROUPBY and HAVING; 2. Decide which record to keep based on business logic, such as using MAX or MIN functions to select the latest or earliest record, and combine sorting, status priority or primary key judgment; 3. Use window functions such as ROW\_NUMBER() to accurately control the deduplication logic, and filter out the records with the highest priority in each group through custom sorting rules to adapt to complex deduplication requirements. Mastering these methods can effectively deal with most duplicate data problems.

Jul 11, 2025 am 12:23 AM

Managing concurrency control with locking in SQL.

Concurrency control ensures database consistency and correctness through locking mechanisms. Common types include shared locks for read operations and exclusive locks for write operations; row-level locks reduce conflicts for high concurrency, table-level locks have poor concurrency capabilities but small overhead; avoiding deadlocks requires unified resource access order, shortening transaction time, reducing data involvement, and selecting appropriate isolation levels to balance consistency and performance.

Jul 10, 2025 pm 01:29 PM

How to use the COALESCE function to handle NULL values in SQL?

TheCOALESCEfunctioninSQLreturnsthefirstnon-NULLvaluefromalistofexpressions,helpingmanagemissingdata.1.ItreplacesNULLsinqueryresultswithdefaultvalues,suchassubstituting'Noemailprovided'foramissingemail.2.Itensuresconsistentformattinginreportsbyreplaci

Jul 10, 2025 pm 01:29 PM