Usage of SQL statement exists and use of substr in Oracle

Transfer required:EXISTS specifies a subquery to detect the presence of a row.

Syntax: EXISTS subquery
Parameter: subquery is a restricted SELECT statement (the COMPUTE clause and the INTO keyword are not allowed).
Result type: Boolean returns TRUE if the subquery contains rows, otherwise returns flase.

The role of not EXISTS is the opposite of EXISTS.

The return value of the EXISTS (including not EXISTS) clause is a bool value. There is a subquery inside the exists (SELECT ... From ...), which I call an inner query statement for exist. The inner query statement returns a result set. The EXISTS clause returns a Boolean value based on the result set of its query statement, either empty or non-empty.

Before inserting a record, you need to check to see if the record already exists, and to perform the insert operation only if the record does not exist, you can prevent the insertion of duplicate records by using the EXISTS conditional clause.
INSERT into Tablein (aname,asex)
SELECT top 1 ' Zhang San ', ' Male ' from Tablein
Where NOT EXISTS (SELECT * from Tablein where Tablein.aid = 7)

The use of exists and in efficiency, usually using exists is higher than in efficiency, because in does not walk the index, but depends on the actual use of:
In the case of large appearance and small inner table, exists is suitable for small appearance and large inner table.

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Simple usage of Oracle's SUBSTR function

SUBSTR (string, intercept start position, intercept length)//return intercepted word

substr (' Hello World ', 0, 1)//return result is ' H ' * start with a string of length 1 from the first character

substr (' Hello World ', 1, 1)//Returns the result for ' H ' *0 and 1 is the first character that represents the start position of the Intercept

substr (' Hello world ', 2,4)//return result as ' Ello '

substr (' Hello world ', -3,3)//Returns the result as ' rld ' * negative (-i) to indicate the start of the intercept is the left number I character of the right end of the string

Test:

Select substr (' Hello world ', -3,3) value from dual;

Attached: Simple usage of substring (INDEX1,INDEX2) in Java

Function: begins to intercept a string of length index2-index1 from a character string index (subscript) of index1.

String str= "Hello World";

System.out.println (str.substring (0,5));

Print Result: Hello

Usage of SQL statement exists and use of substr in Oracle