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Technical Articles

Database

SQL

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

What does union mean in sql?

UNION is an operator in SQL that combines the result sets of two or more SELECT statements and returns a new result set that contains all unique rows. UNION only applies to SELECT statements with the same structure (consistent number of columns and data types) and automatically eliminates duplicate rows.

May 02, 2024 am 04:27 AM

How to use replace function in sql

The REPLACE function in SQL is used to replace all given substrings with another substring in a text or string. Can be used for basic replacement, conditional replacement, and NULL value handling. The basic syntax is: REPLACE(string, old_substring, new_substring).

May 02, 2024 am 04:21 AM

How to use union in sql

The UNION operation combines rows from different tables into a single result set, removing duplicate rows. The syntax format is: SELECT column_list FROM table1 UNION SELECT column_list FROM table2..., where table1 and table2 are the tables to be merged. The UNION operation requires that the participating tables have the same number of columns and data types, and that duplicate rows are removed.

May 02, 2024 am 04:18 AM

Usage of char in sql

The char data type in SQL is a string type used to store fixed-length character data. Each character occupies one byte and is suitable for data that requires a specified length. Its advantages include fixed storage space, convenient comparison and sorting, and improved querying. Performance, but the disadvantage is low storage space utilization and the need to deal with Unicode character set compatibility issues. Unlike varchar, char has a fixed storage space, while varchar allows variable character length and dynamically allocates storage space.

May 02, 2024 am 04:15 AM

The meaning of having in sql

The HAVING clause is used to filter grouped data in a SQL query. Unlike the WHERE clause, which filters individual rows, the HAVING clause is used to filter groups created by the GROUP BY clause. Uses include: filtering summary values based on groups, applying aggregate function conditions, and filtering groups that meet specific conditions.

May 02, 2024 am 04:12 AM

聚合函数

What does having mean in sql

The HAVING clause is used to filter aggregate results in SQL queries and filter rows that meet specific conditions. The HAVING clause can only be used with a GROUP BY clause, and conditions can reference aggregate functions and grouping columns.

May 02, 2024 am 04:09 AM

聚合函数

How to use having in sql

The HAVING clause is used to filter grouped data. The syntax is: SELECT aggregate_functions(column_name) FROM table_name GROUP BY column_name HAVING condition;. It can filter on aggregate functions, grouping columns, constants or operators. For example, the following query filters out groups with at least 3 items in the order and a total amount greater than $100: SELECT COUNT(DISTINCT order_id) AS order_count FROM orders GROUP BY cus

May 02, 2024 am 04:06 AM

聚合函数

Which data in sql needs to be quoted

In SQL, data that needs to be quoted includes character data, date and time values, and NULL values. The purpose of quoting is to prevent SQL injection attacks and to identify the value as text, rather than an SQL command or function. However, when the data type is a number, Boolean, or enumeration value, quoting is not required.

May 02, 2024 am 03:54 AM

What does a character represent in sql?

Characters in SQL are enclosed in single quotes, such as 'A'. Strings are enclosed in double quotes, and characters and strings are different types. Characters enclosed in single quotes are stored unchanged, and strings enclosed in double quotes can contain escape sequences. The single quote character itself can be stored with an escape sequence, such as '\'''.

May 02, 2024 am 03:51 AM

sql语句

What does on mean in sql

In SQL, ON is used to join rows in tables, specifying equal columns in different tables to match and combine rows.

May 02, 2024 am 03:48 AM

The difference between like and = in sql

LIKE and = are operators used for string comparison in SQL, the main difference is that LIKE allows wildcard fuzzy matching (% and _), while = only works for exact matches. LIKE is suitable for fuzzy queries, has slower performance, and cannot use indexes. = is used for exact matching, faster performance, and can use indexes. The choice of operator depends on the specific matching requirements of the query.

May 02, 2024 am 03:42 AM

Usage of like and in in sql

LIKE in SQL is used to find strings containing specific patterns. It supports wildcards, but is not as efficient as the IN operator. IN is used to find fields that match a specified list of values. It is faster and supports index optimization. It is recommended to use IN when looking for specific values, use LIKE when looking for similar strings, and use IN in preference to optimize performance.

May 02, 2024 am 03:39 AM