function
Functions can be roughly divided into the following types:
- Arithmetic functions (functions used to perform numerical calculations)
- String functions (functions used to perform string operations)
- Date function (function used for date manipulation)
- Conversion functions (functions used to convert data types and values)
- Aggregation functions (functions used for data aggregation)
String function
- || ——Splice
String 1 || String 2
|| The function cannot be used in SQL Server and MySQL.
LENGTH- string length LENGTH (string)- LOWER-Lowercase conversion
LOWER (string) - REPLACE-- replacement string
will replace a portion of the string to other strings
REPLACE (target character string, character string before the replacement, the replacement string) - SUBSTRING-string interception
SUBSTRING (the starting position of the object string FROM interception FOR the number of characters intercepted) - UPPER-uppercase conversion
UPPER (string)
Date function
- CURRENT_DATE-Current date The
CURRENT_DATE function can return the date of SQL execution, that is, the date when the function was executed. Because there are no parameters, no parentheses are required - CURRENT_TIME-the current time
CURRENT_TIME function can get the time of SQL execution, which is the time when the function is executed - CURRENT_TIMESTAMP-current date and time
- CAST-type conversion
CAST (value before conversion AS the type of data you want to convert) - COALESCE——Convert NULL to other values
COALESCE (Data 1, Data 2, Data 3 ...)
COALESCE is a SQL-specific function.
LIKE predicate-part of the string consistent query
Partial agreement can be roughly divided into three types: front agreement, middle agreement and rear agreement.
- "%" Is a special symbol for "any string above 0 characters"
- "_" Stands for "any 1 character".
IS NULL, IS NOT NULL-judge whether it is NULL
IN predicate-simple usage of OR
IN (value 1, value 2, value 3, ...) replace multiple OR
NOT IN (value 1, value 2, value 3, ...)
Note:
- NULL data cannot be selected for IN and NOT IN.
- Subqueries can be used as parameters for IN predicates
