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110 lines
3.9 KiB
Markdown
110 lines
3.9 KiB
Markdown
Important changes in sqlpp11
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============================
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Breaking changes in 0.36:
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-------------------------
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__Abstract__:
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One of the main motivations of sqlpp11 is to prevent SQL programming mistakes at compile time. The following changes prevent reported mishaps.
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* `from(a,b)` not allowed anymore, please use explicit joins
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* `where(true)` not allowed anymore, please use `.unconditionally()` or sqlpp::value(true)
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* `some_sql_expression and true` not allowed anymore, please use `tab.col == sqlpp::value(true)` if you really want to express this.
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* `having(expression)` requires `expression` to be made of aggregates, e.g. columns named in `group_by()` or aggregate functions like `count()` or constant values.
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* `where()` and `having` accept only one parameter
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__Explicit joins required__:
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Up until sqlpp11-0.35 you could write something like
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```
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auto result = db(select(all_of(a), all_of(b))
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.from(a,b)
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.where(someCondition));
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```
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Using this syntax in `from()`, it was expected to have the join condition implicitly in the `where()` call.
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But there is no reliable way to tell whether or not that condition is there. Or, if there definitely is none, whether
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it was omitted on purpose or by accident.
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In one case, an accidentally omitted join condition in the `where()` brought a production system to a screeching halt.
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In order to prevent this in the future, sqlpp11 now requires you to join table explicitly, including an explicit join condition, e.g.
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```
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auto result = db(select(all_of(a), all_of(b))
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.from(a.join(b).on(a.b == b.id))
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.where(someCondition));
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```
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Most joins, (`join`/`inner_join`, `left_outer_join`, `right_outer_join` and `outer_join`) require a join condition to given via `on()`.
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The join condition has to be some sqlpp11 boolean expression.
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In those rare cases, when you really need a cross join, you can also use `cross_join()` which has no join condition, of course.
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```
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auto result = db(select(all_of(a), all_of(b))
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.from(a.cross_join(b))
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.where(someCondition));
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```
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__Use `.unconditionally()`__
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If you want to select/update/remove all rows, earlier versions of sqlpp11 required the use of `where(true)`. Since version 0.36, use `unconditionally()`, for instance:
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```
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auto result = db(select(all_of(t)).from(t).unconditionally());
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```
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__Use `sqlpp::value()` to wrap bool values in boolean expressions__
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Lets say you had
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```
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struct X
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{
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int a;
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int b;
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};
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auto x = X{};
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```
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Then earlier versions of sqlpp11 would compile the following expression:
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```
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select(all_of(t)).from(t).where(x.a == x.a or t.b == t.b);
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```
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What you probably meant was:
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```
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select(all_of(t)).from(t).where(t.a == x.a and t.b == x.b);
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```
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In order to prevent this kind of mistake, boolean operators in sql expressions require sqlpp boolean expressions as operators.
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The library also requires the types of the left/right hand side operands of a comparison to be different, so that `t.a < t.a` does not compile any more.
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In the rare case you really have a bool value that you want to use a boolean sql expression, you have to wrap it in sqlpp::value(x), e.g.
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```
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select(all_of(t)).from(t).where(sqlpp::value(x.a == 17) and t.b == x.b);
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```
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__`having()` requires aggregate expressions__
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In older versions, the following code was allowed:
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```
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select(all_of(t)).from(t).where(t.a > 7).having(t.b != "");
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```
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As of sqlpp11-0.36, the having argument must be made of aggregate columns or functions, e.g.
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```
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select(all_of(t)).from(t).unconditionally().group_by(t.b).having(t.b != "", avg(t.c) < 42);
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```
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__`where()` and `having` accept only one expression__
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In older versions, `where()` and `having()` would accept more than one argument and combine those arguments with `and`.
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I am not sure this was ever used. So it just made life harder for the compiler.
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As of version 0.36, `where()` and `having()` accept only one parameter.
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