General MySQL Oracle SQL Server

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Merge Outputs from Two Queries in MySQL
How To Use UNION to Merge Outputs from Two Queries Together in MySQL? If you have two queries that returns the same row fields, you can merge their outputs together with the UNION operator. The following tutorial exercise shows you how to return all links that were created since year 2006 plus the o... 2017-12-21, 2926🔥, 0💬

Violation of Unique Value Constraint in MySQL
What Happens If Unique Value Constraints Are Violated in MySQL? If you are inserting a new record that has values violating a unique constraint, you will get an error. Note that primary key column has a unique value constraint by default. The following tutorial exercise gives you some good examples:... 2018-01-16, 2924🔥, 0💬

Close MySQL Connection Objects in MySQL
How To Close MySQL Connection Objects in MySQL? MySQL connection objects created with mysql_connect() calls should be closed as soon as you have finished all of your database access needs by calling mysql_close($con) function. This will reduce the consumption of connection resources on your MySQL se... 2017-11-11, 2924🔥, 0💬

DML Commands Supported in MySQL
How Many SQL DML Commands Are Supported by "mysql" in MySQL? There are 4 SQL Data Manipulation Language (DML) commands that are supported by "mysql". They are listed below with short descriptions: "INSERT INTO tableName ..." - Inserts new data rows into the specified table. "DELETE FROM tableName ..... 2018-02-08, 2921🔥, 0💬

Using Subqueries in the FROM Clause in MySQL
How To Use Subqueries in the FROM clause in MySQL? If you have a query returning many rows of data, and you want to perform another query on those rows, you can put the first query as a subquery in the FROM clause of the second query. A subquery used in this way become a temporary table, and you mus... 2017-12-21, 2921🔥, 0💬

Experiment with Dead Locks in MySQL
How To Experiment Dead Locks in MySQL? If you want to have some experience with dead locks, you can create two windows running two mysql transactions in two sessions at the REPEATABLE READ transaction isolation level. Then run some UPDATE statements as shown in the tutorial exercise below: (session ... 2017-07-21, 2921🔥, 0💬

JSON_SCHEMA_VALID() - JSON Schema Validation
How to validate a JSON value against a JSON schema using the JSON_SCHEMA_VALID() function? JSON_SCHEMA_VALID(schema, json) is a MySQL built-in function that validates a JSON value against a given JSON schema. For example: SET @schema = '{ "id": "http://json-schema.org/geo", "$schema": "http://json-s... 2024-11-23, 2913🔥, 0💬

Using Subqueries with the EXISTS Operator in MySQL
How To Use Subqueries with the EXISTS Operator in MySQL? A subquery can be used with the EXISTS operator as "EXISTS (subquery)", which returns true if the subquery returns one or more rows. The following statement is a good example of "EXISTS (subquery)". It returns rows from fyi_links table that th... 2017-09-17, 2911🔥, 0💬

Retrieving the Last Sequence ID in MySQL
How To Get the Last ID Assigned by MySQL in MySQL? If you use an ID column with AUTO_INCREMENT attribute, you can use the mysql_insert_id() function to get the last ID value assigned by the MySQL server, as shown in the sample script below: &lt;?php include "mysql_connection.php"; $sql = "INSERT... 2017-09-12, 2911🔥, 0💬

Returning the Second 5 Rows in MySQL
How To Return the Second 5 Rows in MySQL? If you want to display query output in multiple pages with 5 rows per page, and the visitor wants to see the output for the second page, you need to display query output from row 6 to row 10. You can use the "LIMIT startRow maxRows" clause to return only row... 2017-12-21, 2904🔥, 0💬

What Are Group Functions in MySQL
What Are Group Functions in MySQL? Group functions are functions applied to a group of rows. Examples of group functions are: COUNT(*) - Returns the number of rows in the group. MIN(exp) - Returns the minimum value of the expression evaluated on each row of the group. MAX(exp) - Returns the maximum ... 2018-01-06, 2903🔥, 0💬

Entering Numeric Values in MySQL
How To Include Numeric Values in SQL statements in MySQL? If you want to include a numeric value in your SQL statement, you can enter it directly as shown in the following examples: SELECT 255 FROM DUAL; -- An integer 255 SELECT -6.34 FROM DUAL; -- A regular number -6.34 SELECT -32032.6809e+10 FROM ... 2018-03-31, 2901🔥, 0💬

Server Daemon mysqld Administration in MySQL
Where to find answers to frequently asked questions on Server Daemon mysqld Administration in MySQL? Here is a list of frequently asked questions and their answers compiled by FYIcenter.com DBA team on Server Daemon mysqld Administration in MySQL. Clear answers are provided with tutorial exercises o... 2017-12-09, 2898🔥, 0💬

Turn on Binary Logs in MySQL
How To Turn on Binary Logs in MySQL? If you want MySQL to write binary logs to a file, you can use the "--log-bin=fileBaseName" option at the "mysqld" command line. The tutorial exercise below shows you a good example on how to use this option: &gt;cd \mysql\bin &gt;mkdir \mysql\logs &gt... 2017-11-29, 2897🔥, 0💬

Expressions with NULL Values in MySQL
What Happens If NULL Values Are Involved in Expressions in MySQL? If NULL values are used in expressions, the resulting values will be NULL values. In other words: Arithmetic expressions with NULL values result NULL values. Comparison expressions with NULL values result NULL values. Logical expressi... 2018-03-28, 2896🔥, 0💬

What Is InnoDB in MySQL
What Is InnoDB in MySQL? InnoDB is a transaction safe storage engine developed by Innobase Oy (an Oracle company now).   ⇒ What Is BDB in MySQL ⇐ What Is MyISAM in MySQL ⇑ Database Basics and Terminologies in MySQL ⇑⇑ MySQL Database Tutorials 2017-07-03, 2895🔥, 0💬

Insert Multiple Rows with 1 INSERT Statement in MySQL
How To Insert Multiple Rows with One INSERT Statement in MySQL? If you want to insert multiple rows with a single INSERT statement, you can use a subquery instead of the VALUES clause. Rows returned from the subquery will be inserted the target table. The following tutorial exercise gives a good exa... 2018-01-16, 2894🔥, 0💬

Updating Values in a Table in MySQL
How To Update Values in a Table in MySQL? If you want to update some values in one row or multiple rows in a table, you can use the UPDATE statement. The tutorial script below shows a good example: mysql&gt; UPDATE fyi_links SET counts = 999, notes = 'Good.' WHERE id = 101; Query OK, 1 row affec... 2018-01-13, 2894🔥, 0💬

Sorting Output in Descending Order in MySQL
How To Sort Output in Descending Order in MySQL? If you want to sort a column in descending order, you can specify the DESC keyword in the ORDER BY clause. The following SELECT statement first sorts the "tag" in descending order, then sorts the "counts" in ascending order: mysql&gt; SELECT tag, ... 2017-11-02, 2885🔥, 0💬

Number of Rows Selected or Affected in MySQL
How To Get the Number of Rows Selected or Affected by a SQL Statement in MySQL? There are two functions you can use the get the number of rows selected or affected by a SQL statement: mysql_num_rows($res) - Returns the number of rows selected in a result set object returned from SELECT statement. my... 2017-10-08, 2885🔥, 0💬

Execute SQL Statements in MySQL
How To Execute a SQL Statement in MySQL? You can run any types of SQL statements through the mysql_query() function. It takes the SQL statement as a string and returns different types of data depending on the SQL statement type and execution status: Returning FALSE, if the execution failed. Returnin... 2017-10-23, 2881🔥, 0💬

Sort by Multiple Columns in MySQL
Can the Query Output Be Sorted by Multiple Columns in MySQL? You can specifying multiple columns in the ORDER BY clause as shown in the following example statement, which returns employees' salaries sorted by department and salary value: mysql&gt; SELECT tag, counts, url, DATE(created) FROM fyi_... 2017-11-02, 2880🔥, 0💬

Including Comments in SQL Statements in MySQL
How To Include Comments in SQL Statements in MySQL? If you want to include comments in a SQL statement, you can first enter "--", then enter your comment until the end of the line. The tutorial exercise below shows you some good examples: SELECT 'Hello world!' FROM DUAL; -- My first SQL statement! I... 2018-04-12, 2879🔥, 0💬

Turn on Query Logs in MySQL
How To Turn on Query Logs in MySQL? If you want MySQL to write query logs to a file, you can use the "--log=fileName" option at the "mysqld" command line. The tutorial exercise below shows you a good example on how to use this option and view the query log file: &gt;cd \mysql\bin &gt;mkdir \... 2017-12-04, 2877🔥, 0💬

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General MySQL Oracle SQL Server