If Condition In Sql Query Where Clause

We want to do not only write SQL, but also to do a good performance of the SQL, the following for the author to learn, extract, and summarized part of the information to share with you! (1) Select the most efficient table name order (valid only in the Rule-based optimizer): The ORACLE parser processes the table names in the FROM clause in Right-to-left order, and the last table in the FROM clause (the underlying table driving tables) is processed first, In the case where multiple tables are included in the FROM clause, you must select the table with the least number of records as the underlying table. If...

MySQL database sql statement commonly used optimization methods 1. Query optimization, should try to avoid full table scan, should first consider where and order by the columns involved in the establishment of the index. 2. Should be avoided in the where clause on the field null value judgment, otherwise it will cause the engine to abandon the use of indexes and full table scan, such as: select id from t where num is null You can set the default value of num 0, to ensure that Num column table does not null value ...

A few suggestions to improve SQL execution efficiency: Try not to include subqueries in where; queries about time, try not to write: where To_char (dif_date, ' yyyy ') =to_char (' 2007-07-01 ', ' yyyy '); In the filter condition, the condition in which the maximum number of records can be filtered must be placed at the end of the WHERE clause, and the last table (underlying table, driving table) in the FROM clause will be first ...

Hive in the official document of the query language has a very detailed description, please refer to: http://wiki.apache.org/hadoop/Hive/LanguageManual, most of the content of this article is translated from this page, Some of the things that need to be noted during the use process are added. Create tablecreate [EXTERNAL] TABLE [IF not EXISTS] table_name [col_name data_t ...

Hive on Mapreduce Hive on Mapreduce execution Process Execution process detailed parsing step 1:ui (user interface) invokes ExecuteQuery interface, sending HQL query to Driver step 2:driver Create a session handle for the query statement and send the query statement to Compiler for statement resolution and build execution Plan step 3 and 4:compil ...

The cloud storage solution with NoSQL technology is maturing, but the idea of SQL database dominates. This can lead to the use of SQL to resolve NoSQL data modeling issues. Based on the author's Cassandra Project development and project implementation experience, this paper makes some brief guidance on NoSQL modeling. The article does not have the specific syntax guidance data modelling, these questions please refer to the Apache website. Cassandra has been upgraded to the top-level project of the Apache organization. Currently maintain a very fast development upgrade speed. Apache ...

The cloud storage solution with NoSQL technology is maturing, but the idea of SQL database dominates. This can lead to the use of SQL to resolve NoSQL data modeling issues. Based on the author's Cassandra Project development and project implementation experience, this paper makes some brief guidance on NoSQL modeling. The article does not have the specific syntax guidance data modelling, these questions please refer to the Apache website. Cassandra has been upgraded to the top-level project of the Apache organization. Currently maintain a very fast development upgrade speed. Apache ...

Usually when we use the MySQL database, often because of operational errors caused by data loss, MySQL database backup can help us to avoid due to various reasons, data loss or other problems with the database. First, the data backup shortcut Because this method has not been the official official document validation, we tentatively called the experiment. Purpose: Backup hostA host a MySQL database backup TestA, and restored to the hostB machine in the test environment: Operating System: WinNT4.0, Mysql3.22.34, phpM ...