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Technical Articles

Database

SQL

How to add or remove a column from an existing table in SQL?

To add or delete columns in a database table, use the ALTERTABLE statement. 1. Use ALTERTABLE...ADDCOLUMN when adding columns. You need to specify the data type, optional default values ??and constraints, and define locations in a specific database; for example: ALTERTABLEusersADDCOLUMNemailVARCHAR(255)NOTNULLDEFAULT'no_email@example.com'AFTERusername; Note that the default value will affect existing data, the NOTNULL column must have a default value or allow NULL. 2. Use ALTERTABLE...DROPCOLUM to delete the column

Jul 03, 2025 am 01:24 AM

sql database

How Can I Improve the Performance of SQL Queries with Pattern Matching?

ToimproveSQLqueryperformancewithpatternmatching,usethesestrategies:1)Useindexeswiselybystructuringqueriestostartwithfixedstrings,2)Implementfull-textsearchforcomplexpatterns,3)Useregularexpressionsefficiently,4)Limitthequeryscopewithadditionalfilters

Jul 03, 2025 am 01:20 AM

How to join three or more tables in a single SQL query?

To extract data from multiple database tables, it is key to use SQL's JOIN operation, and understanding its logic and syntax is the key. First, JOIN combines data through associated columns, and can continuously use multi-table connections, such as orders, customers and products tables, which can be matched by primary foreign keys; second, common JOIN types include INNERJOIN, LEFTJOIN, RIGHTJOIN and FULLOUTERJOIN, and different types affect the range of the result set; third, JOIN order affects the semantic structure, especially in LEFTJOIN, it should be arranged according to "large to small" to conform to business logic; fourth, AS can be used to set aliases when field conflicts to improve the readability of the query; finally, avoid excessive connections

Jul 03, 2025 am 01:17 AM

How do SQL JOINs work with examples?

SQLJOINscombinedatafrommultipletablesbasedonrelatedcolumns,withdifferenttypesservingspecificpurposes.1)INNERJOINreturnsonlymatchingrowsbetweentables,usefulforstrictlymatcheddata.2)LEFTJOINincludesallrowsfromthelefttableandmatchedrowsfromtheright,fill

Jul 03, 2025 am 01:04 AM

What is a SQL index and how does it improve performance?

ASQLindeximprovesqueryperformancebyprovidingashortcuttolocatedatawithoutscanningtheentiretable.Itworkslikeabookindex,containingasortedlistofvaluesfromoneormorecolumnsandpointingdirectlytocorrespondingrows.1)Indexeshelpwhensearchingforspecificvalues,f

Jul 03, 2025 am 12:59 AM

Performance improvements SQL Index

How to select a random row from a SQL table?

To randomly select a row from a SQL table, it can be achieved through database built-in functions and query techniques. 1. Use ORDERBYRAND() LIMIT1 to be used for MySQL and SQLite. Random numbers are generated through RAND() and the first one is sorted, but the performance is poor when there are large data volumes; 2. PostgreSQL uses the RANDOM() function instead of RAND(); 3. Improve efficiency and random selection can be made based on the primary key range. First get the maximum primary key value, and then generate the first record that is greater than or equal to the random number to query the first record that is greater than or equal to the value to avoid the whole table sorting but may lead to uneven probability; 4. Notes include uneven primary key distribution affecting uniformity, suggesting regular statistical distribution, cache random values, or processing at the program level. According to database type and

Jul 03, 2025 am 12:59 AM

How to insert a new row into a table using SQL?

Common methods of inserting new rows in SQL include: 1. When inserting the entire row of data, the fields must exactly match the table structure, the syntax is INSERTINTOtable\_nameVALUES(...); 2. Inserting data in a specified field is more flexible, allowing some fields to be empty or the default value is used, and the syntax is INSERTINTOtable\_name(column1,column2)VALUES(...); 3. Inserting multiple rows of data at a time can use multiple VALUES combinations in a single statement to improve efficiency. For example, when inserting data into the employees table, you can use full field insertion, specified field insertion and batch insertion methods respectively. Pay attention to the field order and data type matching during operation.

Jul 03, 2025 am 12:42 AM

How do SQL aggregate functions like COUNT and SUM handle NULL values?

SQLaggregatefunctionshandleNULLvaluesbyignoringthem,withkeydifferencesacrossfunctions:1.COUNT(*)countsallrowsincludingNULLs,whileCOUNT(column_name)onlycountsnon-NULLvaluesinthatcolumn;2.SUMskipsNULLvaluesandaddsonlynon-NULLvalues,treatingmissingdataa

Jul 03, 2025 am 12:34 AM

How to use IF/ELSE logic in a SQL SELECT statement?

IF/ELSE logic is mainly implemented in SQL's SELECT statements. 1. The CASEWHEN structure can return different values ??according to the conditions, such as marking Low/Medium/High according to the salary interval; 2. MySQL provides the IF() function for simple choice of two to judge, such as whether the mark meets the bonus qualification; 3. CASE can combine Boolean expressions to process multiple condition combinations, such as judging the "high-salary and young" employee category; overall, CASE is more flexible and suitable for complex logic, while IF is suitable for simplified writing.

Jul 02, 2025 am 01:25 AM

What is the purpose of the DISTINCT keyword in a SQL query?

The DISTINCT keyword is used in SQL to remove duplicate rows in query results. Its core function is to ensure that each row of data returned is unique and is suitable for obtaining a list of unique values ??for a single column or multiple columns, such as department, status or name. When using it, please note that DISTINCT acts on the entire row rather than a single column, and when used in combination with multiple columns, it returns a unique combination of all columns. The basic syntax is SELECTDISTINCTcolumn_nameFROMtable_name, which can be applied to single column or multiple column queries. Pay attention to its performance impact when using it, especially on large data sets that require sorting or hashing operations. Common misunderstandings include the mistaken belief that DISTINCT is only used for single columns and abused in scenarios where there is no need to deduplicate D

Jul 02, 2025 am 01:25 AM

sql distinct

What is a SQL sequence and how is it different from an auto-increment identity?

The main difference between SQL sequence and self-incremental identification lies in scope of action and control flexibility. 1. The sequence is an independent object and can be used in multiple tables, providing more flexible configuration options, such as the start value, step size, maximum value and loop behavior, and requires manual call to generate the next value; 2. Self-increase identification of the columns bound to a specific table, and automatically generates values ??when inserting a new row. The configuration is simple but the control is limited. Selection sequences are suitable for scenarios where cross-tables share counters or require fine control, while self-incremental identification is suitable for simple cases where only a unique ID is required for each table.

Jul 02, 2025 am 01:23 AM

OLTP vs OLAP: Where are they in the cloud?

OLTPandOLAPsystemsinthecloudleveragemanagedserviceslikeAmazonRDSforOLTPandAmazonRedshiftforOLAP,offeringscalabilityandefficiency.1)OLTPusesserviceslikeAmazonRDSforhandlingtransactionswithautomaticscalingandhighavailability.2)OLAPemploysserviceslikeAm

Jul 02, 2025 am 01:22 AM

OLTP OLAP

What is a sequence object in SQL and how is it used?

AsequenceobjectinSQLgeneratesasequenceofnumericvaluesbasedonspecifiedrules,commonlyusedforuniquenumbergenerationacrosssessionsandtables.1.Itallowsdefiningintegersthatincrementordecrementbyasetamount.2.Unlikeidentitycolumns,sequencesarestandaloneandus

Jul 02, 2025 am 01:21 AM

How to create a temporary table in SQL?

Create temporary tables in SQL for storing intermediate result sets. The basic method is to use the CREATETEMPORARYTABLE statement. There are differences in details in different database systems; 1. Basic syntax: Most databases use CREATETEMPORARYTABLEtemp_table (field definition), while SQLServer uses # to represent temporary tables; 2. Generate temporary tables from existing data: structures and data can be copied directly through CREATETEMPORARYTABLEAS or SELECTINTO; 3. Notes include the scope of action is limited to the current session, rename processing mechanism, performance overhead and behavior differences in transactions. At the same time, indexes can be added to temporary tables to optimize

Jul 02, 2025 am 01:21 AM