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Technical Articles

Database

SQL

What are the commands to modify data in sql

The commands to modify data in SQL mainly include the following categories: UPDATE command: update the specified field value in the row that meets the conditions. DELETE command: delete rows that meet the conditions. INSERT command: Inserts new rows into the table. MERGE command: Merge data into a table, supports updates and inserts. TRUNCATE command: Efficiently delete all rows in the table.

May 01, 2024 pm 10:03 PM

How to use sumif in sql

The SUMIF function in SQL is used to sum numeric values under specific conditions. The syntax is: specify the condition and data range to be applied. Use the SUMIF function in SQL queries, noting conditions, ranges, and summing ranges. Run a query to get the sum of the numeric values of cells that meet a condition.

May 01, 2024 pm 10:03 PM

Usage of coalesce function in sql

The COALESCE function in SQL is used to handle NULL values, returning the first non-NULL value in the expression list, or returning the default value if all expressions are NULL. It is mainly used for replacing NULL values, providing default values and cascading checks. To replace NULL values, use COALESCE(expr1, expr2, ...). To provide a default value, use COALESCE(expr1, expr2, ..., default_value). For cascading checks, COALESCE(expr1, expr2, ...) can be used.

May 01, 2024 pm 10:01 PM

Will it be slow to use in to query fields in sql?

Yes, it can be slow in some cases. The IN query compares each row in the table to the value in the subquery, and performance degrades when the subquery result set is large. Factors include subquery size, number of table rows, and indexes. Mitigation measures include limiting subquery results, using concrete indexes, and considering other query types. Best practices are to avoid using IN queries on key columns, use smaller subqueries, add indexes and monitor query plans.

May 01, 2024 pm 10:00 PM

Statements for transaction rollback in sql

Use the ROLLBACK statement to undo changes in an uncommitted transaction and restore the database to the state it was in when the transaction began.

May 01, 2024 pm 10:00 PM

The command in sql to delete records is

The command used to delete records is DELETE, and the syntax is: DELETE FROM table_name WHERE condition; condition specifies which records to delete. Use DELETE FROM table_name directly when deleting unconditionally, and use DELETE FROM table_name WHERE condition when deleting conditionally, such as DELETE FROM table_name. WHERE age > 30 means to delete records with an age greater than 30 in the table.

May 01, 2024 pm 09:57 PM

commands for query in sql

Query commands in SQL are used to retrieve data from the database, the most commonly used command is SELECT, which gets specific data from a table based on WHERE conditions. Other commonly used query commands include INSERT (insert new rows), UPDATE (update existing values), and DELETE (delete rows).

May 01, 2024 pm 09:57 PM

Statements for grouping queries in sql

The statement in SQL used for grouping queries is GROUP BY, which groups a data set by a specified column or expression and calculates the aggregate value of each group, such as sum or average. For example, GROUP BY product_name and SUM(sales_amount) calculates the total sales for each product, producing the following result: product_name, total_sales, where product_name is the grouping column and total_sales is the sum of sales for each group. GROUP BY queries can be nested to create more complex groupings, for example, nested GROUP BY product_ca

May 01, 2024 pm 09:57 PM

聚合函数

How to format the if statement in sql

IF statements are used in SQL to control code execution based on conditions. The formatting rules are as follows: Each keyword occupies one line. Conditional brackets. Code blocks are indented. Statements end with a semicolon. For example: IF (age ≥ 18) THEN Grant access level 1; ELSE Deny access level 1; END IF;

May 01, 2024 pm 09:54 PM

access

Is grouping necessary when using aggregate functions in SQL?

Using aggregate functions in SQL often requires grouping to ensure accuracy of calculations. Grouping can be based on one or more columns, dividing the data into smaller groups and performing aggregate calculations within each group. Groupless aggregation, single-column grouping, and multi-column grouping are all viable options, depending on the aggregation function and grouping requirements.

May 01, 2024 pm 09:54 PM

聚合函数

What statements are used in sql to revoke data operation permissions from users?

The statement in SQL to revoke data operation permissions is REVOKE, and the syntax is: REVOKE <permission> ON <table name> FROM <user>.

May 01, 2024 pm 09:54 PM

How to judge if statement in sql multiple times

Multiple conditions with the THEN and ELSE keywords can be used in SQL to perform multiple judgments, as follows: IF condition1 THEN -- statement block 1ELSIF condition2 THEN -- statement block 2...ELSE -- statement block nEND IF;

May 01, 2024 pm 09:51 PM

What does null mean in sql

A NULL value in SQL means unknown or does not exist, unlike other values such as the empty string or 0. It is not equal to any value, including itself. Methods to identify NULL values include: IS NULL operator and COALESCE() function. You need to be careful when handling NULL values in queries because they may lead to unexpected results. It is recommended to use IS NULL or COALESCE() function to identify NULL values and use the CASE statement. Provide default values and use foreign key constraints to ensure data integrity.

May 01, 2024 pm 09:51 PM

What does floor mean in sql

The FLOOR function in SQL is used to round down a number or expression to the nearest integer less than or equal to the value. It can be used to round floating point numbers, determine date or time boundaries, round monetary values, and perform interval queries or groupings. The FLOOR function is the opposite of the CEIL function, which rounds numbers upward.

May 01, 2024 pm 09:51 PM