comparing null values in sql

Four myths about indexing using indexes: Effects of null values on indexes Let's do some test data first: Sql> CREATE table t (x int, y int); Table created Note that here I do a unique (federated) index of table T: Sql> Create unique index t_idx on T (x,y); Index created

, multiply, divide and so on operation, the result is still empty. Sql> select 1+null from dual; Sql> select 1-null from dual; Sql> select 1*null from dual; Sql> select 1/

; ------------------------------------------------------------------------------- 1 SQL> select 1 from dual where nvl (Null, 0) = nvl (Null, 0 ); ------------------------------------------------------------------------------- 1 2. Null value calculation Add, subtract, multiply, and divide

Label:1.NULL meaning is the missing value (missing value). 2. Three value logic (Three-valued-logic:true,false,unknown). There are three logical predicates in SQL: Ture,false,unknown. In most programming language only True and false in SQL , and all of the existence of unknown is related to null. For example, make the

The concept of null values is a common cause of confusion among novice sql, who often assume that null is the same thing as an empty string. That's not true! For example, the following statements are completely different: Mysql> INSERT into my_table (phone) VALUES (

This article describes how to understand and use the NULL value in MySQL, A friend may refer to the concept of NULL value as a common cause of confusion among SQL beginners. They often think that NULL is the same as an empty string. No! For example, the following statements are completely different: mysql> INSERT IN

; ), -> PARTITION p2 VALUES LESS THAN MAXVALUE ( -> SUBPARTITION s2a, -> SUBPARTITION s2b -> ) -> );Query OK, 0 rows affected (0.04 sec) In future, when the number of purchases for the decade beginning with the year 2000 grows to a point where the default location no longer provides sufficient space, the corresponding rows can be moved using an alter table... reorganize partition stat

MySQL databaseIt is an open source database based on structured data. Most people think that MySQL is the best choice for managing content without transaction processing. SQL statements are the core language of MySQL databases. However, when executing SQL statements in the MySQL database, you need to be careful with two traps. Trap 1: NULL

The MySQL database is an open source database based on structured data. The SQL statement is the core language in the MySQL database. However, in the MySQL database to execute SQL statements, you need to be careful two traps. Trap one: null value is not necessarily empty A null value is a more specific field. In a My

Processing the ten NULL values of MySQ learning notes is a special feature of MySQL. In concept, NULL means "no value" or "unknown value", and it is regarded as a slightly different value. To test NULL, you cannot use arithmetic comparison operators such as =, Mysql> SELECT 1 =

Several issues related to NULL values in MySQL Databases For SQL beginners, the concept of NULL value is often confusing. They often think that NULL is the same as the NULL String. This is not the case. For example, the following

MySQL database is an open source database based on structured data. SQL statements are the core language of MySQL databases. However, when executing SQL statements in the MySQL database, you need to be careful with two traps. Trap 1: NULL values are not necessarily empty A null