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Technical Articles

Database

SQL

How to use insert statement

The INSERT statement can insert new rows into the database table. The syntax is: INSERT INTO table_name (column1, column2, ..., columnN) VALUES (value1, value2, ..., valueN); The steps are as follows: 1. Specify the table name; 2. List the column names into which values are to be inserted; 3. List the corresponding values; 4. End the statement with a semicolon.

May 06, 2024 pm 03:48 PM

Usage of insert statement in SQL

The INSERT statement is used to insert new records into a database table. The syntax is: INSERT INTO table name (column 1, column 2, ...) VALUES (value 1, value 2, ...); the INSERT statement has the following variants: select data insertion from other tables or subqueries, If the primary key conflicts, the existing record will be updated. If the primary key conflicts, the insertion will be ignored. When no column name is specified, values are inserted in the order defined by the table; no value is required for auto-increment columns; the number of affected rows is 1 on successful insertion; an error is thrown on failure.

May 06, 2024 pm 03:45 PM

Is there a difference between null and 0 in sql?

NULL and 0 are different values in SQL: NULL represents an unknown or non-existent value, and 0 represents the numeric value zero. NULL cannot be compared with any value and returns NULL when participating in arithmetic operations; 0 can be compared and participated in arithmetic operations. NULL returns NULL when participating in logical operations, and 0 is considered false. NULL and numeric values are handled differently in database operations, such as in conditional checks.

May 02, 2024 am 05:27 AM

The difference between null and empty in sql

In SQL, NULL represents an unknown or non-existent value. It has its own data type and cannot be indexed. It usually takes up less storage space and is FALSE in Boolean expressions. Null represents an empty string, which has a string data type and can be indexed, considered TRUE. Use NULL to indicate that the data is unknown, and use NULL to indicate that the data is an empty string.

May 02, 2024 am 05:24 AM

mysql

What does a null value represent in mysql?

Null values in MySQL are represented as NULL, indicating that the data does not exist or is unknown. The advantages of using NULL include accuracy, consistency, data integrity, and query optimization, but the disadvantages include potential ambiguity, additional processing, and storage overhead. Other ways to represent null values include the empty string, the number 0, and special characters, but there are data type conversion and ambiguity issues with using these methods.

May 02, 2024 am 05:21 AM

mysql

What does sage mean in sql

SAGE (SQL Anywhere Generalized Extraction) in SQL Anywhere is a feature used to extract and transform data from relational databases for purposes including data extraction, integration, transformation, and reporting.

May 02, 2024 am 05:12 AM

Usage of decimal in sql

The DECIMAL data type in SQL is used to store exact decimal numbers. It has the following syntax: DECIMAL(precision, scale), where precision is the total number of digits and scale is the number of digits after the decimal point. DECIMAL is used to store financial data, monetary values, and other numbers that require high precision. Unlike FLOAT and DOUBLE, DECIMAL stores exact values without using scientific notation. It takes up more storage space than FLOAT or DOUBLE. You should use = and <> operators when comparing. If you need greater precision and range, you can use NUMER

May 02, 2024 am 05:06 AM

Usage of decode in sql

The DECODE function is used to evaluate different values based on a conditional expression, similar to the IF-ELSE syntax. Its usage includes: replacing NULL values, converting data types, and returning different values based on conditions.

May 02, 2024 am 05:00 AM

iphone

begin end usage in sql

In SQL, BEGIN and END are used as transaction boundary markers to ensure the atomicity, durability, and isolation of operations. BEGIN starts a transaction and END commits or rolls back the transaction. 1. BEGIN starts the transaction. 2. Perform an operation (insert, update, etc.). 3. COMMIT commits the transaction and makes the changes permanent. 4. ROLLBACK rolls back the transaction and undoes the changes.

May 02, 2024 am 04:57 AM

并发访问

The main function of rollback statement in sql

The ROLLBACK statement can undo all changes in the current transaction and restore it to the state at the beginning of the transaction. Specific operations include: canceling uncommitted data operations, restoring the original values of modified records, and releasing transaction-locked resources. Use ROLLBACK when an error occurs in a transaction, the user wants to undo changes, or the database needs to be rolled back. It should be noted that ROLLBACK cannot undo DDL operations and can only affect the current transaction. Uncommitted changes will be lost after execution.

May 02, 2024 am 04:54 AM

Usage of avg function in sql

The AVG function in SQL is used to calculate the average of numeric values. Its syntax is AVG(column_name). AVG functions ignore NULL values. You can use the AS keyword to specify aliases for results. It only works for numeric type data, returning NULL if the target column has no value or contains only NULL values, and throwing an error if it contains non-numeric values.

May 02, 2024 am 04:48 AM

What does != mean in sql?

In SQL queries, the != operator means "not equal to" and compares two expressions. If the results are different, it is true, if the results are the same, it is false. Syntax: expression1 != expression2. For example: SELECT * FROM customers WHERE age != 30; This query returns all customer records whose age is not equal to 30.

May 02, 2024 am 04:42 AM

How to express inclusion in sql

The IN operator is used in SQL to express inclusion, and its syntax is "column_name IN (value1, value2, ...)". Extended representations include NOT IN, ANY, and ALL, which check whether a value is not in a list, exists in a subquery, or exists in all subquery rows.

May 02, 2024 am 04:39 AM

What does mod in sql mean?

MOD in SQL is the operator that calculates the remainder. Usage: MOD(x, y), where x is the dividend and y is the divisor. Returns the remainder after dividing x by y, which is always a nonnegative number. Use cases include calculating the day of the week, checking parity, and detecting whether a number is a multiple.

May 02, 2024 am 04:36 AM