What I love about writing SQL Tuning articles is that I very rarely end up publishing the findings I set out to achieve. With this one, I set out to demonstrate the. Other than the requirements for the SET options and table hints, these are the same rules that the Query Optimizer uses to determine whether a table index covers a query. Active Record Query Interface. This guide covers different ways to retrieve data from the database using Active Record. After reading this guide, you will know. Active Record Query Interface Ruby on Rails Guides. If youre used to using raw SQL to find database records, then you will generally find that there are better ways to carry out the same operations in Rails. Update Query Generated By Fetch Query Sql For Version OneActive Record insulates you from the need to use SQL in most cases. Code examples throughout this guide will refer to one or more of the following models All of the following models use id as the primary key, unless specified otherwise. Client lt Application. Record. hasone address. Address lt Application. Record. belongsto client. Order lt Application. Record. belongsto client, countercache true. Role lt Application. Record. hasandbelongstomany clients. Active Record will perform queries on the database for you and is compatible with most database systems, including My. SQL, Maria. DB, Postgre. SQL, and SQLite. Regardless of which database system youre using, the Active Record method format will always be the same. How to get your query results back in CSV, HTML, INSERT, etc formats all in just one step. This work, SQL Anywhere A Holistic Approach to Database Selfmanagement, originally appeared in the Proceedings of the 23rd International Conference on Data. Mysqlquery sends a unique query multiple queries are not supported to the currently active database on the server thats associated with the specified link. The only proper PDO tutorial. Why PDO Connecting. DSN Running queries. PDOquery Prepared statements. Protection from SQL injections Binding methods. This is MySQL PHP tutorial. In this tutorial, you will learn the basics of database programming in MySQL and PHP language. Retrieving Objects from the Database. To retrieve objects from the database, Active Record provides several finder methods. Each finder method allows you to pass arguments into it to perform certain queries on your database without writing raw SQL. The methods are findcreatewithdistincteagerloadextendingfromgrouphavingincludesjoinsleftouterjoinslimitlocknoneoffsetorderpreloadreadonlyreferencesreorderreverseorderselectwhere. Finder methods that return a collection, such as where and group, return an instance of Active. Record Relation. Methods that find a single entity, such as find and first, return a single instance of the model. The primary operation of Model. Convert the supplied options to an equivalent SQL query. Fire the SQL query and retrieve the corresponding results from the database. Instantiate the equivalent Ruby object of the appropriate model for every resulting row. Run afterfind and then afterinitialize callbacks, if any. Retrieving a Single Object. Active Record provides several different ways of retrieving a single object. Using the find method, you can retrieve the object corresponding to the specified primary key that matches any supplied options. For example. Find the client with primary key id 1. Client. find1. 0. Client id 1. 0, firstname Ryan. The SQL equivalent of the above is. SELECT FROM clients WHERE clients. LIMIT 1. The find method will raise an Active. Record Record. Not. Found exception if no matching record is found. You can also use this method to query for multiple objects. Call the find method and pass in an array of primary keys. The return will be an array containing all of the matching records for the supplied primary keys. For example. Find the clients with primary keys 1 and 1. Client. find1, 1. Or even Client. find1, 1. Client id 1, firstname Lifo, lt Client id 1. Ryan. The SQL equivalent of the above is. SELECT FROM clients WHERE clients. IN 1,1. 0. The find method will raise an Active. Record Record. Not. Found exception unless a matching record is found for all of the supplied primary keys. The take method retrieves a record without any implicit ordering. For example. client Client. Client id 1, firstname Lifo. The SQL equivalent of the above is. SELECT FROM clients LIMIT 1. The take method returns nil if no record is found and no exception will be raised. You can pass in a numerical argument to the take method to return up to that number of results. For example. client Client. Client id 1, firstname Lifo. Client id 2. 20, firstname Sara. The SQL equivalent of the above is. SELECT FROM clients LIMIT 2. The take method behaves exactly like take, except that it will raise Active. Record Record. Not. Found if no matching record is found. The retrieved record may vary depending on the database engine. The first method finds the first record ordered by primary key default. For example. client Client. Client id 1, firstname Lifo. The SQL equivalent of the above is. SELECT FROM clients ORDER BY clients. ASC LIMIT 1. The first method returns nil if no matching record is found and no exception will be raised. If your default scope contains an order method, first will return the first record according to this ordering. You can pass in a numerical argument to the first method to return up to that number of results. For example. client Client. Client id 1, firstname Lifo. Client id 2, firstname Fifo. Client id 3, firstname Filo. The SQL equivalent of the above is. SELECT FROM clients ORDER BY clients. ASC LIMIT 3. On a collection that is ordered using order, first will return the first record ordered by the specified attribute for order. Client. order firstname. Client id 2, firstname Fifo. The SQL equivalent of the above is. SELECT FROM clients ORDER BY clients. ASC LIMIT 1. The firstActive. Record Record. Not. Found if no matching record is found. The last method finds the last record ordered by primary key default. For example. client Client. Client id 2. 21, firstname Russel. The SQL equivalent of the above is. SELECT FROM clients ORDER BY clients. DESC LIMIT 1. The last method returns nil if no matching record is found and no exception will be raised. If your default scope contains an order method, last will return the last record according to this ordering. You can pass in a numerical argument to the last method to return up to that number of results. For example. client Client. Client id 2. 19, firstname James. Client id 2. 20, firstname Sara. Client id 2. 21, firstname Russel. The SQL equivalent of the above is. SELECT FROM clients ORDER BY clients. DESC LIMIT 3. On a collection that is ordered using order, last will return the last record ordered by the specified attribute for order. Client. order firstname. Client id 2. 20, firstname Sara. The SQL equivalent of the above is. SELECT FROM clients ORDER BY clients. DESC LIMIT 1. The last Active. Record Record. Not. Found if no matching record is found. The findby method finds the first record matching some conditions. For example. Client. Lifo. lt Client id 1, firstname Lifo. Client. findby firstname Jon. It is equivalent to writing. Client. wherefirstname Lifo. The SQL equivalent of the above is. SELECT FROM clients WHERE clients. Lifo LIMIT 1. The findby Active. Record Record. Not. Found if no matching record is found. For example. Client. Active. Record Record. Not. Found. This is equivalent to writing. Client. wherefirstname does not exist. Retrieving Multiple Objects in Batches. We often need to iterate over a large set of records, as when we send a newsletter to a large set of users, or when we export data. This may appear straightforward. This may consume too much memory if the table is big. User. all. each do user. News. Mailer. weeklyuser. But this approach becomes increasingly impractical as the table size increases, since User. Active Record to fetch the entire table in a single pass, build a model object per row, and then keep the entire array of model objects in memory. Indeed, if we have a large number of records, the entire collection may exceed the amount of memory available. Rails provides two methods that address this problem by dividing records into memory friendly batches for processing. The first method, findeach, retrieves a batch of records and then yields each record to the block individually as a model. SAVEPOINT to UPDATE, 3 of 8. Purpose. Use a SELECT statement or subquery to retrieve data from one or more tables, object tables, views, object views, or materialized views. Note If the result or part of the result of a SELECT statement is equivalent to an existing materialized view, then Oracle may use the materialized view in place of one or more tables specified in the SELECT statement. This substitution is called query rewrite, and takes place only if cost optimization is enabled and the QUERYREWRITEENABLED parameter is set to TRUE. To determine whether query write has occurred, use the EXPLAINPLAN statement. Additional Topics. Prerequisites. For you to select data from a table or materialized view, the table or materialized view must be in your own schema or you must have the SELECT privilege on the table or materialized view. For you to select rows from the base tables of a view,You must have the SELECT privilege on the view, and. Whoever owns the schema containing the view must have the SELECT privilege on the base tables. The SELECTANYTABLE system privilege also allows you to select data from any table or any materialized view or any views base table. To issue a flashback query using the flashbackclause, either you must have FLASHBACK object privilege on the objects in the select list, or you must have FLASHBACKANYTABLE system privilege. Syntaxselect Text description of selectforupdateclause subquery Text description of subquerysubqueryfactoringclause, selectlist, tablereference, hierarchicalqueryclause, groupbyclause, orderbyclause subqueryfactoringclause Text description of subqueryfactoringclauseselectlist Text description of selectlisttablereference Text description of tablereferencequerytableexpression, flashbackclause flashbackclause Text description of flashbackclausequerytableexpression Text description of querytableexpressionsubqueryrestrictionclause, tablecollectionexpression sampleclause Text description of sampleclausesubqueryrestrictionclause Text description of subqueryrestrictionclausetablecollectionexpression Text description of tablecollectionexpressionjoinedtable Text description of joinedtabletablereference jointype Text description of jointypewhereclause Text description of whereclausehierarchicalqueryclause Text description of hierarchicalqueryclausegroupbyclause Text description of groupbyclauserollupcubeclause, groupingsetsclause rollupcubeclause Text description of rollupcubeclausegroupingexpressionlist groupingsetsclause Text description of groupingsetsclauserollupcubeclause, groupingexpressionlist groupingexpressionlist Text description of groupingexpressionlistexpressionlist Text description of expressionlistorderbyclause Text description of orderbyclauseforupdateclause Text description of forupdateclause. Semanticssubqueryfactoringclause. The subqueryfactoringclause WITHqueryname lets you assign names to subquery blocks. You can then reference the subquery block multiple places in the query by specifying the query name. Oracle optimizes the query by treating the query name as either an inline view or as a temporary table. You can specify this clause in any top level SELECT statement and in most types of subqueries. The query name is visible to all subsequent subqueries except the subquery that defines the query name itself and to the main query. Restrictions on Subquery Factoring. You cannot nest this clause. That is, you cannot specify the subqueryfactoringclause as a subquery within another subqueryfactoringclause. In a query with set operators, the set operator subquery cannot contain the subqueryfactoringclause, but the FROM subquery can contain the subqueryfactoringclause. Specify a comment that passes instructions to the optimizer on choosing an execution plan for the statement. DISTINCT UNIQUESpecify DISTINCT or UNIQUE if you want Oracle to return only one copy of each set of duplicate rows selected these two keywords are synonymous. Duplicate rows are those with matching values for each expression in the select list. Restrictions on DISTINCT and UNIQUE Queries. When you specify DISTINCT or UNIQUE, the total number of bytes in all select list expressions is limited to the size of a data block minus some overhead. This size is specified by the initialization parameter DBBLOCKSIZE. You cannot specify DISTINCT if the selectlist contains LOB columns. ALLSpecify ALL if you want Oracle to return all rows selected, including all copies of duplicates. The default is ALL. asteriskSpecify the asterisk to select all columns from all tables, views, or materialized views listed in the FROM clause. Note If you are selecting from a table that is, you specify a table in the FROM clause rather than a view or a materialized view, then then columns that have been marked as UNUSED by the ALTERTABLESETUNUSED statement are not selected. The selectlist lets you specify the columns you want to retrieve from the database. For queryname, specify a name already specified in the subqueryfactoringclause. You must have specified the subqueryfactoringclause in order to specify queryname in the selectlist. If you specify queryname in the selectlist, then you also must specify queryname in the querytableexpression FROM clause. Specify the object name followed by a period and the asterisk to select all columns from the specified table, view, or materialized view. A query that selects rows from two or more tables, views, or materialized views is a join. You can use the schema qualifier to select from a table, view, or materialized view in a schema other than your own. If you omit schema, then Oracle assumes the table, view, or materialized view is in your own schema. Specify an expression representing the information you want to select. A column name in this list can be qualified with schema only if the table, view, or materialized view containing the column is qualified with schema in the FROM clause. If you specify a member method of an object type, then you must follow the method name with parentheses even if the method takes no arguments. Specify a different name alias for the column expression. Oracle will use this alias in the column heading. The AS keyword is optional. The alias effectively renames the select list item for the duration of the query. The alias can be used in the orderbyclause, but not other clauses in the query. Restrictions on the Select List. If you also specify a groupbyclause in this statement, then this select list can contain only the following types of expressions. Constants. Aggregate functions and the functions USER, UID, and SYSDATEExpressions identical to those in the groupbyclause. Expressions involving the preceding expressions that evaluate to the same value for all rows in a group. You can select a rowid from a join view only if the join has one and only one key preserved table. The rowid of that table becomes the rowid of the view. If two or more tables have some column names in common, then you must qualify column names with names of tables. FROM Clause. The FROM clause lets you specify the objects from which data is selected. Use the querytableexpression clause to identify a table, view, materialized view, or partition, or to specify a subquery that identifies the objects.