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Technical Articles

Database

How to get the week number from a date in SQL

To get the number of weeks from dates, different functions must be used according to the database system, because each system has different definitions of the week's start date and the first week; MySQL uses WEEK() or WEEKOFYEAR(), PostgreSQL uses EXTRACT (WEEKFROMdate) to support ISO weeks by default, SQLServer uses DATEPART (WEEK, date) to start from Sunday by default, and ISO weeks can be obtained through DATEPART (ISO_WEEK, date). Oracle uses TO_CHAR (date,'IW'), SQLite needs to use strftime ('%W', date) and no native ISO

Sep 06, 2025 am 03:45 AM

sql date function

How to view binary data in Navicat?

ToviewandworkwithbinarydatainNavicat,youcanusebuilt-intoolsandmethodsdependingontheformatandpurpose.1.Double-clickthebinaryfieldtoopenthedataviewerandswitchbetweenHexorTextdisplaymodesforinspection.2.ExportBLOBdatatodiskwiththecorrectfileextensionfor

Sep 06, 2025 am 03:43 AM

navicat binary data

What is a composite primary key in MySQL?

AcompositeprimarykeyinMySQLusesmultiplecolumnstouniquelyidentifyarow,suchas(student_id,course_id)inanenrollmentstable,wherethecombinationensuresuniquenessbecauseneithercolumnalonecan;thisenforcesNOTNULLconstraintsonbothcolumns,createsasingleclustered

Sep 06, 2025 am 03:03 AM

How to Use Transactions in MongoDB

MongoDB supports multi-document transactions since version 4.0 and needs to be used in replica sets or sharded cluster environments; when using it, transactions must be started through a session, operations must be performed and submissions or rollbacks are properly handled. At the same time, transactions should be kept short, high concurrency scenarios should be avoided, and command restrictions and timeout control should be paid attention to.

Sep 06, 2025 am 02:38 AM

How to use STRING_AGG in SQL

STRING_AGG is a SQL aggregate function for concatenating multiple rows of values ??into a single string, supporting specified delimiters and optional sorting. Its basic syntax is STRING_AGG (expression, delimiter) and can be combined with ORDERBY or WITHINGROUP control order. It is often used to merge related data such as employee names, labels, etc. in GROUPBY queries. This function automatically ignores NULL values ??and supports custom delimiters such as commas, arrows or newlines. It is suitable for SQLServer2017, PostgreSQL, etc., but MySQL uses GROUP_CONCAT, and Oracle uses LISTAGG. When using it, you need to pay attention to correct grouping and explicit sorting to ensure the results

Sep 06, 2025 am 02:34 AM

sql

How to insert data from one table to another in SQL?

To insert data from one table into another table, you should use the INSERTINTO...SELECT statement, 1. Ensure that the target table already exists and the column data types are compatible, 2. Use the basic syntax of INSERTINTO target table (column name) SELECT corresponding column FROM source table WHERE conditions, 3. You can filter specific rows through WHERE, 4. If the target table has fewer columns, only matching columns are selected, 5. Pay attention to the database specific syntax when inserting across databases; this method does not create a new table. If you need to create a table at the same time, you can use SELECTINTO or CREATETABLE...ASSELECT. In the end, you must ensure that the constraints and data types of the target table can accept input data.

Sep 06, 2025 am 02:26 AM

How to install phpMyAdmin using Docker

First, make sure to install Docker and DockerCompose, then use docker-compose.yml to configure MySQL and phpMyAdmin services, then start the container through docker-composeup-d, and finally access http://localhost:8080 in the browser and log in with the root account and set password to complete the installation and access of phpMyAdmin. It is recommended to create a dedicated user and restrict port exposure to improve security.

Sep 06, 2025 am 02:01 AM

docker

How to use the COUNT function in SQL

COUNT(*) returns the total number of rows in the table, regardless of whether there is a NULL value; COUNT(column_name) only calculates the number of non-NULL values ??in the specified column; COUNT(DISTINCTcolumn_name) calculates the number of unique non-NULL values ??in the column; combined with the WHERE clause, conditional counting can be achieved, and GROUPBY can be used to count by group and filter the results with HAVING. Correct selection of COUNT form and avoid common errors can effectively improve the accuracy of data query and ultimately achieve efficient data analysis.

Sep 06, 2025 am 01:03 AM

How to Optimize Aggregation Pipelines in MongoDB

Filterdataearlyusing$matchand$projecttoreducedocumentvolumeandsize.2.Use$limitand$sortefficientlywithindexesandtop-ksorting.3.Optimize$lookupbyfilteringinsidethepipelineandindexingforeignfields.4.LeverageindexesandexplainplanstominimizeCOLLSCANandavo

Sep 06, 2025 am 12:34 AM

What are the different types of joins in Oracle?

ThemaintypesofjoinsinOracleare:1.INNERJOINreturnsonlymatchingrowsfrombothtables;2.LEFTJOINreturnsallrowsfromthelefttableandmatchedrowsfromtheright,withNULLsforunmatchedright-sidecolumns;3.RIGHTJOINreturnsallrowsfromtherighttableandmatchedrowsfromthel

Sep 05, 2025 am 08:17 AM

How to use prepared statements in MySQL

Using preprocessing statements can effectively prevent SQL injection and improve performance. The answer is to separate SQL structure and data to achieve safe and efficient query execution. 1. In MySQL native commands, use PREPARE, SET, EXECUTE and DEALLOCATE statements to define and execute preprocessing statements, such as PREPAREstmt_nameFROM'SELECT*FROMusersWHEREid=?'; 2. In PHP's MySQLi, use prepare() to create a statement, bind_param() to bind parameters, execute() to execute, and finally close the statement; 3. In PHP's PDO, support naming placeholders such as:id,

Sep 05, 2025 am 08:04 AM

How to find the mode (most frequent value) in SQL?

TofindthemodeinSQL,firstgroupbythecolumnandcountthefrequencyofeachvalue.2.Then,identifythemaximumfrequencyusingasubqueryorwindowfunctiontofilterforthemostfrequentvalue(s).3.UseasubqueryapproachforcompatibilityacrossallSQLdialectsorwindowfunctionslike

Sep 05, 2025 am 07:41 AM

How to find all tables with a specific column name in SQL?

To find all tables containing specific column names, it can be done by querying the system metadata table, the most common method is to use INFORMATION_SCHEMA.COLUMNS. 1. In standard SQL, execute SELECTTABLE_NAMEFROMINFORMATION_SCHEMA.COLUMNSWHERECOLUMN_NAME='your_column_name' to return all table names in the specified column. If you need to define a specific schema or database, you can add the TABLE_SCHEMA or TABLE_CATALOG conditions. 2. In SQLServer, you can use sys.columns and sys.ta

Sep 05, 2025 am 07:13 AM

sql 查找表

How to force a query to use a specific index in MySQL

USEINDEXsuggestsanindexbutallowsMySQLtoignoreitifatablescanisbetter;2.FORCEINDEXrequirestheuseofaspecificindexandpreventstablescans,whichcanimproveperformancewhentheoptimizermakespoorchoicesbutmaydegradeperformanceifmisused;3.IGNOREINDEXpreventsMySQL

Sep 05, 2025 am 06:53 AM