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- Usage of coalesce in sql
- The COALESCE function returns the specified non-null value or, if all values are NULL, the specified default value. Usage: 1. Get the first non-null value; 2. Provide a default value; 3. Check multiple values in cascade. Note: Only the first non-null value is returned to ensure that the default value type is consistent. This function avoids NULL values in the query, ensuring that meaningful values are returned.
- SQL 950 2024-04-29 13:12:16
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- The role of check in mysql
- CHECK constraints in MySQL are used to impose more complex restrictions on columns or expressions to maintain data integrity. It allows administrators to define conditions that ensure column values meet specific criteria. CHECK constraints can be added when creating a table or added to an existing table through the ALTER TABLE statement, whose syntax is: CHECK (expression). Benefits include improved data integrity, limiting invalid input, providing better performance, and simplifying application logic. It is important to note that complex CHECK constraints may impact performance, and they do not work with virtual columns, so triggers can be used as an alternative to CHECK constraints in some cases.
- Mysql Tutorial 1086 2024-04-29 05:21:17
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- What does field mean in mysql?
- The FIELD keyword in MySQL is used to find the sequence number of a column in a table. By specifying an expression and the column name to be compared, the index of the matching column can be returned. Otherwise, 0 or NULL is returned. It is often used in dynamic SQL and determining column order.
- Mysql Tutorial 962 2024-04-29 05:15:23
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- What does from mean in mysql?
- FROM is a keyword in a MySQL query that specifies the data source (table or view). It is used in a SELECT statement to identify the data participating in the query and ensure that the data returned is relevant to the specified data source.
- Mysql Tutorial 683 2024-04-29 05:12:14
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- Usage of enum in mysql
- The ENUM data type in MySQL is used to store a predefined collection of values, allowing only options within a specified range. ENUM can be defined explicitly (CREATE TABLE) or implicitly (INSERT). Using ENUM ensures data integrity, space savings, and ease of querying. But it has the problem of limited value set, and querying a large number of values may be slow.
- Mysql Tutorial 869 2024-04-29 05:09:15
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- How to use on in mysql
- The ON clause is used to specify the conditions for comparing and merging rows of two tables in JOIN operations, including inner joins, left outer joins, right outer joins, and full outer joins. The ON condition uses only equality comparisons, you can use multiple conditions to specify more complex join rules, and can be used with other JOIN types.
- Mysql Tutorial 776 2024-04-29 05:00:25
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- What does round mean in mysql?
- The ROUND function is used to round a number to the specified number of digits: Syntax: ROUND(number, decimals); when decimals is a positive value, round to the specified number of decimal places; when decimals is a negative value, round to the specified integer digits; decimals is 0 When , it is rounded to the nearest integer; when number or decimals is NULL, NULL is returned.
- Mysql Tutorial 910 2024-04-29 04:57:12
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- Usage of as in mysql
- The AS keyword in MySQL is used to create an alias for a table or a temporary name for a column in a query to replace the original name with a more understandable name. It improves query readability, disambiguates, and creates temporary names. The usage syntax is: SELECT ... AS alias_name FROM ... WHERE ....
- Mysql Tutorial 1014 2024-04-29 04:51:15
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- What does as mean in mysql
- AS in MySQL is used to create table or column aliases to simplify the reference of tables or columns in queries: Table alias: When using multiple tables in a query, specify a table alias to make it more convenient to reference columns in the table. Column alias: Specify a column alias to use a different name or alias in query results.
- Mysql Tutorial 1041 2024-04-29 04:48:15
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- What does asc mean in mysql
- ASC means ascending order in MySQL and works for numeric and string types. Use the ASC keyword followed by the column name in a SELECT statement to sort data in ascending order, as opposed to DESC. ASC is the default sort order and if not specified, data will be sorted in ascending order.
- Mysql Tutorial 1011 2024-04-29 04:45:21
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- Can the length of varchar in mysql not be set?
- No, the VARCHAR data type in MySQL must specify a length to define its maximum number of characters. If the length is not specified, MySQL will throw an error.
- Mysql Tutorial 1052 2024-04-29 04:42:12
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- The difference between varchar and char in mysql
- The main difference between the VARCHAR and CHAR data types in MySQL is the storage method: CHAR is stored as a fixed length, while VARCHAR is stored based on the actual string length. CHAR is suitable for fixed-length strings, and index search and comparison operations are faster, but storage space is wasted; VARCHAR is suitable for variable-length strings, saving storage space, but index search and comparison operations are slower. Data length, performance, and storage efficiency should be considered when selecting a data type.
- Mysql Tutorial 851 2024-04-29 04:39:15
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- The role of where in mysql
- The WHERE clause is used to filter MySQL query results based on conditions and include rows that meet the conditions in the results. It performs conditional filtering by specifying expressions, such as value comparison, range comparison, or logical operations, and supports complex queries and acquisition of specific data.
- Mysql Tutorial 1371 2024-04-29 04:36:14
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- What does where in mysql mean?
- WHERE clause, used in MySQL to filter rows based on specific criteria. 1. Structure: SELECT <columns> FROM <table_name> WHERE <condition> 2. Usage: Specify filter conditions and use relational operators to compare column values, constants or other column values. 3. Function: Retrieve specific data, reduce processing time, optimize data operations and combine queries.
- Mysql Tutorial 508 2024-04-29 04:33:15
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- How to write grouping function in mysql
- The grouping function in MySQL is used to calculate aggregate values by grouping a data set. Commonly used functions are: SUM: Calculate the sum of the values in the specified column COUNT: Calculate the number of non-NULL values in the specified column AVG: Calculate the average value of the values in the specified column MIN: Calculate the minimum value in the specified column MAX: Calculate the number of non-NULL values in the specified column the maximum value of
- Mysql Tutorial 1169 2024-04-29 04:30:23
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