23 KiB
Database: Query Builder
- Introduction
- Running Database Queries
- Select Statements
- Raw Expressions
- Joins
- Basic Where Clauses
- Ordering, Grouping, Limit & Offset
- Insert Statements
- Update Statements
- Delete Statements
- Pessimistic Locking
Introduction
TinyORM's database query builder provides a convenient, fluent interface to creating and running database queries. It can be used to perform most database operations in your application.
The TinyORM query builder uses QSqlQuery parameter binding to protect your application against SQL injection attacks. There is no need to clean or sanitize strings passed to the query builder as query bindings.
{note} QSqlQuery does not support binding column names. Therefore, you should never allow user input, to dictate the column names referenced by your queries, including "order by" columns.
Running Database Queries
Retrieving All Rows From A Table
You may use the table method provided by the DB facade to begin a query. The table method returns a fluent query builder instance for the given table, allowing you to chain more constraints onto the query and then finally retrieve the results of the query using the get method:
#include <orm/db.hpp>
// Log a list of all of the application's users
auto [ok, query] = DB::table("users")->get();
while (query.next())
qDebug() << "id :" << query.value("id").toULongLong() << ";"
<< "name :" << query.value("name").toString();
The get method returns a std::tuple<bool, QSqlQuery> containing the results of the query where each result can be accessed by QSqlQuery::next method, look into the QSqlQuery documentation how to obtain results from the "query". You may access each column's value by QSqlQuery::value method. The first bool return value is the value returned from QSqlQuery::exec method:
#include <QDebug>
#include <orm/db.hpp>
auto [ok, users] = DB::table("users")->get();
while(users.next())
qDebug() << users.value("name").toString();
Retrieving A Single Row / Column From A Table
If you just need to retrieve a single row from a database table, you may use the QueryBuilder::first method. This method will return a QSqlQuery object, on which was internally called QSqlQuery::first method. This method retrieves the first record in the result, if available, and positions the query on the retrieved record:
auto [ok, user] = DB::table("users")->whereEq("name", "John").first();
user.value("email").toString();
If you don't need an entire row, you may extract a single value from a record using the value method. This method will return the value of the column directly as QVariant:
auto email = DB::table("users")->whereEq("name", "John").value("email").toString();
To retrieve a single row by its id column value, use the find method. This method retrieves the first record in the result, if available, and positions the query on the retrieved record:
auto [ok, user] = DB::table("users")->find(3);
user.value("email").toString();
Select Statements
Specifying A Select Clause
You may not always want to select all columns from a database table. Using the select method, you can specify a custom "select" clause for the query:
#include <orm/db.hpp>
auto [ok, users] = DB::table("users")
->select({"name", "email as user_email"})
.get();
The distinct method allows you to force the query to return distinct results:
auto [ok, users] = DB::table("users")->distinct().get();
If you already have a query builder instance and you wish to add a column to its existing select clause, you may use the addSelect method:
auto query = DB::table("users")->select("name");
auto [ok, users] = query.addSelect("age").get();
Raw Expressions
Sometimes you may need to insert an arbitrary string into a query. To create a raw string expression, you may use the raw method provided by the DB facade:
auto [ok, users] = DB::table("users")
->select(DB::raw("count(*) as user_count, status"))
.where("status", "<>", 1)
.groupBy("status")
.get();
{note} Raw statements will be injected into the query as strings, so you should be extremely careful to avoid creating SQL injection vulnerabilities.
Joins
Inner Join Clause
The query builder may also be used to add join clauses to your queries. To perform a basic "inner join", you may use the join method on a query builder instance. The first argument passed to the join method is the name of the table you need to join to, while the remaining arguments specify the column constraints for the join. You may even join multiple tables in a single query:
#include <orm/db.hpp>
auto [ok, users] = DB::table("users")
->join("contacts", "users.id", "=", "contacts.user_id")
.join("orders", "users.id", "=", "orders.user_id")
.select({"users.*", "contacts.phone", "orders.price"})
.get();
Left Join / Right Join Clause
If you would like to perform a "left join" or "right join" instead of an "inner join", use the leftJoin or rightJoin methods. These methods have the same signature as the join method:
auto [ok, users] = DB::table("users")
->leftJoin("posts", "users.id", "=", "posts.user_id")
.get();
auto [ok, users] = DB::table("users")
->rightJoin("posts", "users.id", "=", "posts.user_id")
.get();
Advanced Join Clauses
You may also specify more advanced join clauses. To get started, pass a closure as the second argument to the join method. The closure will receive a Orm::Query::JoinClause instance which allows you to specify constraints on the "join" clause:
#include <orm/db.hpp>
#include <orm/query/joinclause.hpp>
DB::table("users")
->join("contacts", [](auto &join)
{
join.on("users.id", "=", "contacts.user_id")
.orOn(...);
})
.get();
If you would like to use a "where" clause on your joins, you may use the where and orWhere methods provided by the Orm::Query::JoinClause instance. Instead of comparing two columns, these methods will compare the column against a value:
DB::table("users")
->join("contacts", [](auto &join)
{
join.on("users.id", "=", "contacts.user_id")
.where("contacts.user_id", ">", 5);
})
.get();
Basic Where Clauses
Where Clauses
You may use the query builder's where method to add "where" clauses to the query. The most basic call to the where method requires three arguments. The first argument is the name of the column. The second argument is an operator, which can be any of the database's supported operators. The third argument is the value to compare against the column's value.
For example, the following query retrieves users where the value of the votes column is equal to 100 and the value of the age column is greater than 35:
auto [ok, users] = DB::table("users")
->where("votes", "=", 100)
.where("age", ">", 35)
.get();
For convenience, if you want to verify that a column is = to a given value, you may call whereEq method. Similar XxxEq methods are also defined for other commands:
auto [ok, users] = DB::table("users")->whereEq("votes", 100).get();
As previously mentioned, you may use any operator that is supported by your database system:
auto [ok, users] = DB::table("users")
->where("votes", ">=", 100)
.get();
auto [ok, users] = DB::table("users")
->where("votes", "<>", 100)
.get();
auto [ok, users] = DB::table("users")
->where("name", "like", "T%")
.get();
You may also pass a QVector<Orm::WhereItem> of conditions to the where function. Each Orm::WhereItem structure should contain the four arguments typically passed to the where method:
auto [ok, users] = DB::table("users")
->where({
{"status", 1}, // "=" by default
{"subscribed", 1, "<>"},
}).get();
Or Where Clauses
When chaining together calls to the query builder's where method, the "where" clauses will be joined together using the and operator. However, you may use the orWhere or orWhereEq method to join a clause to the query using the or operator. The orWhere method accepts the same arguments as the where method:
auto [ok, users] = DB::table("users")
->where("votes", ">", 100)
.orWhere("name", "=", "John")
.orWhereEq("name", "Jack")
.get();
If you need to group an "or" condition within parentheses, you may pass a closure as the first argument to the orWhere method:
auto [ok, users] = DB::table("users")
->where("votes", ">", 100)
.orWhere([](auto &query) {
query.whereEq("name", "Abigail")
.where("votes", ">", 50);
})
.get();
The example above will produce the following SQL:
select * from users where votes > 100 or (name = "Abigail" and votes > 50)
Condition Operator Overriding
The where method overload with a QVector<Orm::WhereItem> as the first argument joins conditions using the and operator by default:
auto [ok, users] = DB::table("users")
->where({
{"first_name", "John"},
{"votes", 50, ">"},
}).get();
Conditions operator can be overridden by the fourth argument in the Orm::WhereItem structure:
auto [ok, users] = DB::table("users")
->where({
{"first_name", "John"},
{"votes", 50, ">", "or"},
}).get();
Or by the second where argument, in this case all conditions will be joined by this condition, but it is still possible to override them by the fourth argument in the Orm::WhereItem structure:
auto [ok, users] = DB::table("users")
->where({
{"first_name", "John"},
{"last_name", "Smith"},
{"votes", 50, ">", "and"},
}, "or")
.get();
The example above will produce the following SQL:
select * from users where (first_name = "John" or last_name = "Smith" and votes > 50)
{tip} Still, it is a better idea to use Logical Grouping described few lines below, which allows better control of the parentheses.
Additional Where Clauses
whereIn / whereNotIn / orWhereIn / orWhereNotIn
The whereIn method verifies that a given column's value is contained within the given QVector<QVariant>:
auto [ok, users] = DB::table("users")
->whereIn("id", {1, 2, 3})
.get();
The whereNotIn method verifies that the given column's value is not contained in the given QVector<QVariant>:
auto [ok, users] = DB::table("users")
->whereNotIn("id", {1, 2, 3})
.get();
whereNull / whereNotNull / orWhereNull / orWhereNotNull
The whereNull method verifies that the value of the given column is NULL:
auto [ok, users] = DB::table("users")
->whereNull("updated_at")
.get();
The whereNotNull method verifies that the column's value is not NULL:
auto [ok, users] = DB::table("users")
->whereNotNull("updated_at")
.get();
whereColumn / orWhereColumn
The whereColumnEq method may be used to verify that two columns are equal:
auto [ok, users] = DB::table("users")
->whereColumnEq("first_name", "last_name")
.get();
You may also pass a comparison operator to the whereColumn method:
auto [ok, users] = DB::table("users")
->whereColumn("updated_at", ">", "created_at")
.get();
You may also pass a QVector<Orm::WhereColumnItem> of column comparisons to the whereColumn method. These conditions will be joined using the and operator:
auto [ok, users] = DB::table("users")
->whereColumn({
{"first_name", "last_name"},
{"updated_at", "created_at", ">"},
}).get();
Conditions operator can also be overridden by the fourth argument in the Orm::WhereColumnItem structure:
auto [ok, users] = DB::table("users")
->whereColumn({
{"first_name", "last_name"},
{"updated_at", "created_at", ">", "or"},
}).get();
Or by the second whereColumn argument:
auto [ok, users] = DB::table("users")
->whereColumn({
{"first_name", "last_name"},
{"updated_at", "created_at", ">"},
}, "or")
.get();
Logical Grouping
Sometimes you may need to group several "where" clauses within parentheses in order to achieve your query's desired logical grouping. In fact, you should generally always group calls to the orWhere method in parentheses in order to avoid unexpected query behavior. To accomplish this, you may pass a closure to the where method:
auto [ok, users] = DB::table("users")
->where("name", "=", "John")
.where([](auto &query) {
query.where("votes", ">", 100)
.orWhere("title", "=", "Admin");
})
.get();
As you can see, passing a closure into the where method instructs the query builder to begin a constraint group. The closure will receive a query builder instance which you can use to set the constraints that should be contained within the parenthesis group. The example above will produce the following SQL:
select * from users where name = "John" and (votes > 100 or title = "Admin")
Ordering, Grouping, Limit & Offset
Ordering
The orderBy Method
The orderBy method allows you to sort the results of the query by a given column. The first argument accepted by the orderBy method should be the column you wish to sort by, while the second argument determines the direction of the sort and may be either asc or desc:
auto [ok, users] = DB::table("users")
->orderBy("name", "desc")
.get();
To sort by multiple columns, you may simply invoke orderBy as many times as necessary:
auto [ok, users] = DB::table("users")
->orderBy("name", "desc")
.orderBy("email", "asc")
.get();
The latest & oldest Methods
The latest and oldest methods allow you to easily order results by date. By default, the result will be ordered by the table's created_at column. Or, you may pass the column name that you wish to sort by:
auto [ok, users] = DB::table("users")
->latest()
.first();
Removing Existing Orderings
The reorder method removes all of the "order by" clauses that have previously been applied to the query:
auto query = DB::table("users")->orderBy("name");
auto [ok, unorderedUsers] = query.reorder().get();
You may pass a column and direction when calling the reorder method in order to remove all existing "order by" clauses and apply an entirely new order to the query:
auto query = DB::table("users")->orderBy("name");
auto [ok, usersOrderedByEmail] = query.reorder("email", "desc").get();
Grouping
The groupBy & having Methods
As you might expect, the groupBy and having methods may be used to group the query results. The having method's signature is similar to that of the where method:
auto [ok, users] = DB::table("users")
->groupBy("account_id")
.having("account_id", ">", 100)
.get();
You may pass multiple items to the groupBy method to group by multiple columns:
auto [ok, users] = DB::table("users")
->groupBy({"first_name", "status"})
.having("account_id", ">", 100)
.get();
Limit & Offset
The skip & take Methods
You may use the skip and take methods to limit the number of results returned from the query or to skip a given number of results in the query:
auto [ok, users] = DB::table("users")->skip(10).take(5).get();
Alternatively, you may use the limit and offset methods. These methods are functionally equivalent to the take and skip methods, respectively:
auto [ok, users] = DB::table("users")
->offset(10)
.limit(5)
.get();
Insert Statements
The query builder also provides an insert method that may be used to insert records into the database table. The insert method accepts the QVariantMap of column names and values:
DB::table("users")->insert({
{"email", "kayla@example.com"},
{"votes", 0},
});
You may insert several records at once by passing a QVector<QVariantMap>. Each QVariantMap represents a record that should be inserted into the table:
DB::table("users")->insert({
{{"email", "picard@example.com"}, {"votes", 0}},
{{"email", "janeway@example.com"}, {"votes", 0}},
});
The insertOrIgnore method will ignore duplicate record errors while inserting records into the database:
DB::table("users")->insertOrIgnore({
{{"id", 1}, {"email", "sisko@example.com"}},
{{"id", 2}, {"email", "archer@example.com"}},
});
Auto-Incrementing IDs
If the table has an auto-incrementing id, use the insertGetId method to insert a record and then retrieve the ID:
auto id = DB::table("users")->insertGetId({
{"email", "john@example.com"},
{"votes", 0},
});
Update Statements
In addition to inserting records into the database, the query builder can also update existing records using the update method. The update method, accepts a QVector<Orm::UpdateItem> of column and value pairs, indicating the columns to be updated. You may constrain the update query using where clauses:
auto [affected, query] = DB::table("users")
->whereEq("id", 1)
.update({{"votes", 1}});
Increment & Decrement
The query builder also provides convenient methods for incrementing or decrementing the value of a given column. Both of these methods accept at least one argument: the column to modify. A second argument may be provided to specify the amount by which the column should be incremented or decremented:
DB::table("users")->increment<int>("votes");
DB::table("users")->increment("votes", 5);
DB::table("users")->decrement<int>("votes");
DB::table("users")->decrement("votes", 5.2); // float or double type
You may also specify additional columns to update during the operation as a QVector<Orm::UpdateItem>:
DB::table("users")->increment("votes", 1, {{"name", "John"}});
You should constrain increment, decrement by where to update only specific record in the database, otherwise a column in all records will be modified.
DB::table("users")->whereEq("id", 1).increment("votes", 5);
Delete Statements
The query builder's remove, or an alias deleteRow method may be used to delete records from the table. You may constrain delete statements by adding "where" clauses before calling the delete method:
DB::table("users")->remove();
DB::table("users")->where("votes", ">", 100).remove();
{note} 'delete' can not be used as the method name because it is the reserved word.
You may also pass record id to the remove method as the first argument, it is the shortcut method, which internally calls where:
DB::table("users")->remove(2);
Truncate Statement
If you wish to truncate an entire table, which will remove all records from the table and reset the auto-incrementing ID to zero, you may use the truncate method:
DB::table("users")->truncate();
Pessimistic Locking
The query builder also includes a few functions to help you achieve "pessimistic locking" when executing your select statements. To execute a statement with a "shared lock", you may call the sharedLock method. A shared lock prevents the selected rows from being modified until your transaction is committed:
DB::table("users")
->where("votes", ">", 100)
.sharedLock()
.get();
Alternatively, you may use the lockForUpdate method. A "for update" lock prevents the selected records from being modified or from being selected with another shared lock:
DB::table("users")
->where("votes", ">", 100)
.lockForUpdate()
.get();