8.6 KiB
Introduction
This page explains select statements with a static structure. If you want to learn about constructing select statements at runtime, you should still read this page first and then move on to dynamic select statements.
Lets assume we have a table representing
CREATE TABLE foo (
id bigint,
name varchar(50),
hasFun bool
);
(This is SQL for brevity, not C++, see here for details on how to define types representing the tables and columns you want to work with)
Lets also assume we have an object db representing a connection to your database.
A Basic example
This shows how you can select some data from table and iterate over the results:
for (const auto& row : db(select(foo.name, foo.hasFun)
.from(foo)
.where(foo.id > 17 and foo.name.like("%bar%"))))
{
if (row.name.is_null())
std::cerr << "name is null" << std::endl;
else
std::string name = row.name; // string-like fields are implicitly convertible to string
bool hasFun = row.hasFun; // bool fields are implicitly convertible to bool
}
So, what's happening here? Lets ignore the gory details for a moment. Well, there is a select statement.
select(foo.name, foo.hasFun)
.from(foo)
.where(foo.id > 17 and foo.name.like("%bar%"))
It selects two columns name and hasFun from table foo for rows which match the criteria given in the where condition. That's about as close to real SQL as it can get...
The select expression is fed into the call operator of the connection object db. This method sends the select statement to the database and returns an object representing the results. In the case of select statements, the result object represents zero or more rows.
One way of accessing the rows is to iterate over them in a range-based for loop.
for (const auto& row : ...)
Ok, so the variable row is an object that represents a single result row. You really want to use auto here, because you don't want to write down the actual type. Trust me. But the wonderful thing about the row object is that it has appropriately named and typed members representing the columns you selected. This is one of the utterly cool parts of this library.
The Select Statement
Select
The select method takes zero or more named expression arguments.
Named expressions are expressions with a name. No surprise there. But what kind of expressions have a name? Table columns, for instance. In our example, that would be foo.id, foo.name and foo.hasFun. Most function calls also result in named expressions, like count(foo.id).
So what about unnamed expressions? Results of binary operators like (foo.id + 17) * 4 have no name. But you can give them a name using the as(alias) method. The easiest way is to use a named expression as alias, for instance ((foo.id + 17) * 4).as(foo.id), e.g.
for (const auto& row : db(select(((foo.id + 17) * 4).as(foo.id)).from(tab)))
{
std::cout << row.id << std::endl;
}
Another option is to define an alias like this:
SQLPP_ALIAS_PROVIDER(total);
for (const auto& row : db(select(sum(id).as(total)).as(foo.id)).from(tab)))
{
std::cout << row.total << std::endl;
}
Using aliases also comes in handy when you join tables and have several columns of the same name, because no two named expressions in a select must have the same name. So if you want to do something like
select(foo.id, bar.id); // compile error
One of the columns needs an alias.
SQLPP_ALIAS_PROVIDER(barId);
select(foo.id, bar.id.as(barId));
Select Columns
All examples above called the select() function with one or more arguments, but select() can also be called with no arguments. In that case, the selected columns have to be added afterwards
sqlpp::select().columns(foo.id, foo.name);
See also dynamic select statements.
Select Flags
The following flags are currently supported:
- sqlpp::all
- sqlpp::distinct
Flags are added via the flags() method:
sqlpp::select().flags(sqlpp::all).columns(foo.id, foo.name);
or
select(foo.id, foo.name).flags(sqlpp::all);
The latter is shorter than the former, but the former is closer to SQL syntax and probably easier to read.
Sub-Select
A select statement with one column also is named expression. This means you can use one select as a sub-select column of another select. For example:
SQLPP_ALIAS_PROVIDER(cheese_cake); // Declared outside of function
// ...
for (const auto& row : db(
select(all_of(foo),
select(sum(bar.value)).from(bar).where(bar.id > foo.id)),
select(bar.value.as(cheese_cake)).from(bar).where(bar.id > foo.id))
.from(foo)))
{
const int x = row.id;
const int64_t a = row.sum;
const int b = row.cheese_cake;
}
The name of the sub select is the name of the one column. If required, you can rename it using as(), as usual.
Select All Columns
Statements like SELECT * from foo is used pretty often in SQL. sqlpp11 offers something similar:
select(all_of(foo));
From
The from method expects one argument. The following subsections expand on the types of valid arguments:
- tables
- tables with an alias (via the
asmethod) - sub-selects with an alias
- joins
Tables
This is the most simple case.
select(all_of(foo)).from(foo);
Aliased Tables
Table aliases are useful in self-joins.
SQLPP_ALIAS_PROVIDER(left);
SQLPP_ALIAS_PROVIDER(right);
auto l = foo.as(left);
auto r = foo.as(right);
select(all_of(l)).from(l.join(r).on(l.x == r.y));
Aliased tables might also be used to increase the readability of generated SQL code, for instance if you have very long table names.
Aliased Sub-Select
A select can be used as a pseudo table in another select. You just have to give it a name.
SQLPP_ALIAS_PROVIDER(sub);
auto sub_select = select(all_of(foo)).from(foo).as(sub);
The variable sub_select can be used as a table now.
Joins
You can join two tables like this:
foo.join(bar).on(foo.id == bar.foo);
If you want to join more tables, you can chain joins.
foo.join(bar).on(foo.id == bar.foo).left_outer_join(baz).on(bar.id == baz.ref);
Hint: Omitting the call to on will result in mildly horrible error messages...
Where
The where condition can be set via the where method, which takes a boolean expression argument, for instance:
select(all_of(foo)).from(foo).where(foo.id != 17 and foo.name.like("%cake"));
Group By
The method group_by takes one or more expression arguments, for instance:
select(all_of(foo)).from(foo).group_by(foo.name);
Having
The having condition can be set via the having method, just like the where method.
Order By
The order_by method takes one of more order expression, which are normal expression adorned with .asc() or .desc(), e.g.
select(all_of(foo)).from(foo).order_by(foo.name.asc());
Limit And Offset
The methods limit and offset take a size_t argument, for instance:
select(all_of(foo)).from(foo).limit(10u).offset(20u);
For Update
The for_update method modifies the query with a simplified "FOR UPDATE" clause without columns.
select(all_of(foo)).from(foo).where(foo.id != 17).for_update();
Running The Statement
OK, so now we know how to create a select statement. But the statement does not really do anything unless we hand it over to the database:
db(select(all_of(foo)).from(foo));
This call returns a result object of a pretty complex type. Thus, you would normally want to use auto:
auto result = db(select(all_of(foo)).from(foo));
Accessing The Results
The result object created by executing a select query is a container of result rows.
Range-based For Loops
Not surprisingly, you can iterate over the rows using a range-based for-loop like this:
for (const auto& row : db(select(all_of(foo)).from(foo)))
{
std::cerr << row.id << std::endl;
std::cerr << row.name << std::endl;
}
Lovely, isn't it? The row objects have types specifically tailored for the select query you wrote. You can access their member by name, and these members have the expected type.
Function-based Access
If for some reason, you don't want to use range-based for-loops, you can use front() and pop_front() on the result, like this:
while(!result.empty())
{
const auto& row = result.front();
std::cerr << row.id << std::endl;
std::cerr << row.name << std::endl;
result.pop_front();
}