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Technical Articles

Database

Oracle

How to use coalesce function in oracle

The Oracle COALESCE function returns the first non-null value in a set of expressions and is commonly used in data processing and repairing missing values. Usage: 1. Determine the expression to be checked; 2. Specify the COALESCE function; 3. Provide the expression to be checked. This function accepts any number of expressions as parameters and returns the first non-null value, or NULL if all expressions are null.

Apr 30, 2024 am 07:36 AM

oracle

How to delete table in oracle

To delete an Oracle table, perform the following steps: 1. Connect to the database; 2. Execute the DROP TABLE command and specify the table name; Note: Deleting a table will permanently delete its data. Please ensure that the table is no longer needed and update the table that depends on it. database object.

Apr 30, 2024 am 07:33 AM

oracle

The difference between blob and clob in oracle

The main differences between BLOB (Binary Large Object) and CLOB (Character Large Object) in Oracle are: 1. BLOB stores binary data, while CLOB stores character data; 2. The maximum size of BLOB is 4 GB, and the maximum size of CLOB is 4 GB or database block 32 times the size; 3. BLOB does not support indexes, while CLOB supports part of the index; 4. Querying BLOB will load the entire BLOB, while querying CLOB only loads the first 4000 characters; 5. BLOB is stored in the LOB segment, while CLOB

Apr 30, 2024 am 07:30 AM

oracle

Usage of substr in oracle

The SUBSTR function is used to extract a substring from a string. The syntax is SUBSTR(string, start, length). You need to specify the source string, starting position and length (optional), and the function extracts the substring of the specified length starting from the specified position. For example, SUBSTR('Hello World', 1, 5) returns "Hello" and SUBSTR('Hello World', 7) returns "World".

Apr 30, 2024 am 07:24 AM

oracle

date usage in oracle

The DATE data type stores date values that do not contain time information. Its uses include saving date information, date operations, and data type conversion. Example usages include: calculating date intervals, comparing dates, and extracting date components. Note that the DATE type does not store time zone information, which ranges from January 1, 4712 BC to December 31, 9999 AD.

Apr 30, 2024 am 07:21 AM

oracle

The difference between inner join and outer join in oracle

The connection types in Oracle are divided into inner joins and outer joins. Inner joins return results for matching rows only, while outer joins return matching rows and rows that appear in only one table. There are three types of outer joins: left outer join (returns all rows from the left table), right outer join (returns all rows from the right table), and full outer join (returns all rows from both tables). The characteristic of inner join is to match rows. The left outer join uses NULL to fill the null values of the right table, the right outer join uses NULL to fill the left table null values, and the full outer join uses NULL to fill the null values of both sides of the table.

Apr 30, 2024 am 07:18 AM

oracle

How to use join on in oracle

The JOIN ON operator in Oracle is used to join tables based on common columns. Its syntax is: SELECT ... FROM table name 1 JOIN table name 2 ON table name 1.common column = table name 2.common column. This operator can be used to join multiple tables and supports other comparison operators and AND operators.

Apr 30, 2024 am 07:15 AM

oracle

How to use union all in oracle

UNION ALL is used in Oracle to merge result sets from different tables or subqueries while retaining duplicate rows. The specific usage is as follows: merge rows in different tables: SELECT FROM table 1 UNION ALL SELECT FROM table 2 merge duplicate rows in the same table: SELECT FROM table UNION ALL SELECT FROM table

Apr 30, 2024 am 07:09 AM

oracle

How to use union in oracle

The usage of UNION in Oracle is to merge multiple query result sets with the same structure into a single result set. This operator removes duplicate records unless UNION ALL is used, which merges all records, including duplicates.

Apr 30, 2024 am 07:06 AM

oracle

length usage in oracle

The LENGTH function returns the number of characters in a string. It accepts CHAR, VARCHAR2, NCHAR, and NVARCHAR2 data types as input, ignores whitespace, and returns an integer result.

Apr 30, 2024 am 07:00 AM

oracle

casewhen usage in oracle

The CASE WHEN statement in Oracle is used to return different values based on conditions. Syntax: CASE WHEN condition THEN result ELSE default_result END. Usage: 1. Condition check: WHEN clause contains the condition and returned result; 2. Default result: ELSE clause specifies the default result when any condition is not met. Example: Return income_level based on salary value: salary > 5000: high income; 3000 ≤ salary ≤ 5000: middle income; salary < 3000: low income.

Apr 30, 2024 am 06:57 AM

oracle

How to use sum function in oracle

The SUM function calculates the sum of a series of values in Oracle. Syntax: SUM(expression), where expression is a numeric expression or column name. It can sum all row values in a column, or sum rows that meet a specific condition via a WHERE clause. The SUM function returns a value of type NUMBER and returns a positive number even if the argument is negative. It supports advanced usage, such as with GROUP BY and HAVING clauses, and as a window function.

Apr 30, 2024 am 06:54 AM

oracle

How to use having in oracle

The HAVING clause is used to filter the result set grouped by the GROUP BY clause. Its syntax is HAVING <condition>, where <condition> is a Boolean expression. The difference with the WHERE clause is that the HAVING clause filters groups after aggregation, while the WHERE clause filters rows before aggregation. It can be used to filter grouped result sets, perform aggregate calculations on data, create hierarchical reports, or summarize queries.

Apr 30, 2024 am 06:51 AM

oracle 聚合函数

How to use truncate in oracle

The TRUNCATE command in Oracle is used to quickly delete all data in the table. It does not use transaction logs, is fast, and cannot be rolled back. It will reallocate the table space, retain the table definition, and reset the identity column. Compared with DELETE, TRUNCATE is faster, cannot be rolled back, and space will be reallocated.

Apr 30, 2024 am 06:48 AM

oracle