ejbql5.html

j2eePDF格式的电子书

</div>
</td>
    <td><a name="wp80342"> </a><div class="pCellBody">
T
</div>
</td>
    <td><a name="wp80344"> </a><div class="pCellBody">
T
</div>
</td>
</tr>
  <tr align="left">    <td><a name="wp80346"> </a><div class="pCellBody">
F
</div>
</td>
    <td><a name="wp80348"> </a><div class="pCellBody">
T
</div>
</td>
    <td><a name="wp80350"> </a><div class="pCellBody">
F
</div>
</td>
    <td><a name="wp80352"> </a><div class="pCellBody">
U
</div>
</td>
</tr>
  <tr align="left">    <td><a name="wp80354"> </a><div class="pCellBody">
U
</div>
</td>
    <td><a name="wp80356"> </a><div class="pCellBody">
T
</div>
</td>
    <td><a name="wp80358"> </a><div class="pCellBody">
U
</div>
</td>
    <td><a name="wp80360"> </a><div class="pCellBody">
U
</div>
</td>
</tr>
</table>
</div>
</li></div>
</ul></div>
<a name="wp80361"> </a><h4 class="pHeading3">
Equality Semantics
</h4>
<a name="wp80363"> </a><p class="pBody">
In EJB QL, only values of the same type can be compared. However, this rule has one exception: Exact and approximate numeric values can be compared. In such a comparison, the required type conversion adheres to the rules of Java numeric promotion.
</p>
<a name="wp80364"> </a><p class="pBody">
EJB QL treats compared values as if they were Java types, not as if they represented types in the underlying data store. For example, if a persistent field could be either an integer or a <code class="cCode">NULL</code>, then it must be designated as an <code class="cCode">Integer</code> object, not as an <code class="cCode">int</code> primitive. This designation is required because a Java object can be <code class="cCode">NULL</code> but a primitive cannot.
</p>
<a name="wp80365"> </a><p class="pBody">
Two strings are equal only if they contain the same sequence of characters. Trailing blanks are significant; for example, the strings <code class="cCode">&#39;abc&#39;</code> and <code class="cCode">&#39;abc &#39;</code> are not equal.
</p>
<a name="wp80366"> </a><p class="pBody">
Two entity beans of the same abstract schema type are equal only if their primary keys have the same value.</p><div align="left">
<table border="1" summary="NOT Operator Logic" id="wp80372">
  <caption><a name="wp80372"> </a><div class="pTableTitle">
Table 24-8   NOT Operator Logic&nbsp;
</div>
</caption>
  <tr align="center">    <th><a name="wp80376"> </a><div class="pCellHeading">
NOT
</div>
</th>
    <th><a name="wp80378"> </a><div class="pCellHeading">
&#160;
</div>
</th>
</tr>
  <tr align="left">    <td><a name="wp80380"> </a><div class="pCellBody">
T
</div>
</td>
    <td><a name="wp80382"> </a><div class="pCellBody">
F
</div>
</td>
</tr>
  <tr align="left">    <td><a name="wp80384"> </a><div class="pCellBody">
F
</div>
</td>
    <td><a name="wp80386"> </a><div class="pCellBody">
T
</div>
</td>
</tr>
  <tr align="left">    <td><a name="wp80388"> </a><div class="pCellBody">
U
</div>
</td>
    <td><a name="wp80390"> </a><div class="pCellBody">
U
</div>
</td>
</tr>
</table>
</div>
<p class="pBody"></p><div align="left">
<table border="1" summary="Conditional Text" id="wp80396">
  <caption><a name="wp80396"> </a><div class="pTableTitle">
Table 24-9   Conditional Test&nbsp;
</div>
</caption>
  <tr align="center">    <th><a name="wp80404"> </a><div class="pCellHeading">
Conditional Test
</div>
</th>
    <th><a name="wp80406"> </a><div class="pCellHeading">
T 
</div>
</th>
    <th><a name="wp80408"> </a><div class="pCellHeading">
F 
</div>
</th>
    <th><a name="wp80410"> </a><div class="pCellHeading">
U 
</div>
</th>
</tr>
  <tr align="left">    <td><a name="wp80412"> </a><div class="pCellBody">
Expression <code class="cCode">IS TRUE</code>
</div>
</td>
    <td><a name="wp80414"> </a><div class="pCellBody">
T
</div>
</td>
    <td><a name="wp80416"> </a><div class="pCellBody">
F
</div>
</td>
    <td><a name="wp80418"> </a><div class="pCellBody">
F
</div>
</td>
</tr>
  <tr align="left">    <td><a name="wp80420"> </a><div class="pCellBody">
Expression <code class="cCode">IS FALSE</code>
</div>
</td>
    <td><a name="wp80422"> </a><div class="pCellBody">
F
</div>
</td>
    <td><a name="wp80424"> </a><div class="pCellBody">
T
</div>
</td>
    <td><a name="wp80426"> </a><div class="pCellBody">
F
</div>
</td>
</tr>
  <tr align="left">    <td><a name="wp80428"> </a><div class="pCellBody">
Expression is unknown
</div>
</td>
    <td><a name="wp80430"> </a><div class="pCellBody">
F
</div>
</td>
    <td><a name="wp80432"> </a><div class="pCellBody">
F
</div>
</td>
    <td><a name="wp80434"> </a><div class="pCellBody">
T
</div>
</td>
</tr>
</table>
</div>
<p class="pBody">
</p>
<a name="wp80436"> </a><h3 class="pHeading2">
SELECT Clause
</h3>
<a name="wp80437"> </a><p class="pBody">
The <code class="cCode">SELECT</code> clause defines the types of the objects or values returned by the query. The <code class="cCode">SELECT</code> clause has the following syntax:
</p>
<div class="pPreformattedRelative"><pre class="pPreformattedRelative">
select_clause ::= <code class="cCodeBold">SELECT</code> [<code class="cCodeBold">DISTINCT</code> ] {select_expression 
    |<code class="cCodeBold">OBJECT</code>( identification_variable) }

select_expression ::= single_valued_path_expression |
    aggregate_select_expression

aggregate_select_expression ::=
    {<code class="cCodeBold">AVG</code> |<code class="cCodeBold">MAX</code> |<code class="cCodeBold">MIN</code> |<code class="cCodeBold">SUM</code> |<code class="cCodeBold">COUNT</code> }( [<code class="cCodeBold">DISTINCT</code> ]
    cmp_path_expression) |
    <code class="cCodeBold">COUNT</code> ( [<code class="cCodeBold">DISTINCT</code> ] identification_variable |
    single_valued_cmr_path_expression)<a name="wp80438"> </a>
</pre></div>
<a name="wp80440"> </a><h4 class="pHeading3">
Return Types
</h4>
<a name="wp80442"> </a><p class="pBody">
The return type defined by the <code class="cCode">SELECT</code> clause must match that of the finder or select method for which the query is defined. 
</p>
<a name="wp80443"> </a><p class="pBody">
For finder method queries, the return type of the <code class="cCode">SELECT</code> clause is the abstract schema type of the entity bean that defines the finder method. This abstract schema type maps to either a remote or local interface. If the bean's remote home interface defines the finder method, then the return type is the remote interface (or a collection of remote interfaces). Likewise, if the local home interface defines the finder method, the return type is the local interface (or a collection). For example, the <code class="cCode">LocalPlayerHome</code> interface of the <code class="cCode">PlayerEJB</code> entity bean defines the <code class="cCode">findall</code> method:
</p>
<div class="pPreformattedRelative"><pre class="pPreformattedRelative">
public Collection findAll() throws FinderException;<a name="wp80445"> </a>
</pre></div>
<a name="wp80446"> </a><p class="pBody">
The EJB QL query of the <code class="cCode">findall</code> method returns a collection of <code class="cCode">LocalPlayer</code> interface types:
</p>
<div class="pPreformattedRelative"><pre class="pPreformattedRelative">
SELECT OBJECT(p) 
FROM Player p<a name="wp80447"> </a>
</pre></div>
<a name="wp80449"> </a><p class="pBody">
For select method queries (except for aggregate function queries), the return type of the <code class="cCode">SELECT</code> clause may be one of the following:
</p>
<div class="pSmartList1"><ul class="pSmartList1">
<a name="wp80451"> </a><div class="pSmartList1"><li>The abstract schema of the entity bean that contains the select method</li></div>
<a name="wp80452"> </a><div class="pSmartList1"><li>The abstract schema of a related entity bean</li></div>
<a name="wp80453"> </a><p class="pBodyRelative">
(By default, each of these abstract schema types maps to the local interface of the entity bean. Although uncommon, in the deployment descriptor you may override the default mapping by specifying a remote interface.)
</p>
<a name="wp80454"> </a><div class="pSmartList1"><li>A persistent field</li></div>
</ul></div>
<a name="wp80455"> </a><p class="pBody">
The <code class="cCode">PlayerEJB</code> entity bean, for example, implements the <code class="cCode">ejbSelectSports</code> method, which returns a collection of <code class="cCode">String</code> objects for <code class="cCode">sport</code>. The <code class="cCode">sport</code> is a persistent field of the <code class="cCode">LeagueEJB</code> entity bean. See <a  href="EJBQL4.html#wp79876">Example 11&nbsp;</a>.
</p>
<a name="wp80459"> </a><p class="pBody">
A <code class="cCode">SELECT</code> clause cannot specify a collection-valued expression. For example, the <code class="cCode">SELECT</code> clause <code class="cCode">p.teams</code> is invalid because <code class="cCode">teams</code> is a collection. However, the clause in the following query is valid because the <code class="cCode">t</code> is a single element of the <code class="cCode">teams</code> collection:
</p>
<div class="pPreformattedRelative"><pre class="pPreformattedRelative">
SELECT t
FROM Player p, IN (p.teams) AS t<a name="wp80460"> </a>
</pre></div>
<a name="wp81788"> </a><p class="pBody">
For select method queries with an aggregate function (<code class="cCode">AVG</code>, <code class="cCode">COUNT</code>, <code class="cCode">MAX</code>, <code class="cCode">MIN</code>, <code class="cCode">SUM</code>), in the <code class="cCode">SELECT</code> clause, the following rules apply:
</p>
<div class="pSmartList1"><ul class="pSmartList1">
<a name="wp81792"> </a><div class="pSmartList1"><li>The select method must return a single object, primitive, or wrapper type that is compatible with the standard JDBC conversion mappings for the persistent field type.</li></div>
<a name="wp81796"> </a><div class="pSmartList1"><li>For the <code class="cCode">AVG</code>, <code class="cCode">MAX</code>, <code class="cCode">MIN</code>, <code class="cCode">SUM </code>functions, if the select method return type is an object and the function returns no values, then the select method returns null. In this case, if the select method return type is a primitive, then the container throws the <code class="cCode">ObjectNotFoundException</code>.</li></div>
<a name="wp81794"> </a><div class="pSmartList1"><li>For the <code class="cCode">COUNT</code> function, the result of the select method must be an exact numeric type. If the function returns no values, the select method returns 0.</li></div>
</ul></div>
<a name="wp80462"> </a><h4 class="pHeading3">
DISTINCT and OBJECT Keywords
</h4>
<a name="wp80463"> </a><p class="pBody">
The <code class="cCode">DISTINCT</code> keyword eliminates duplicate return values. If the method of the query returns a <code class="cCode">java.util.Collection</code>--which allows duplicates--then you must specify the <code class="cCode">DISTINCT</code> keyword to eliminate duplicates. However, if the method returns a <code class="cCode">java.util.Set</code>, the <code class="cCode">DISTINCT</code> keyword is redundant because a <code class="cCode">java.util.Set</code> may not contain duplicates. 
</p>
<a name="wp80464"> </a><p class="pBody">
The <code class="cCode">OBJECT</code> keyword must precede a stand-alone identification variable, but it must not precede a single-valued path expression. If an identification variable is part of a single-valued path expression, it is not stand-alone.
</p>
<a name="wp81187"> </a><h4 class="pHeading3">
Aggregate Functions
</h4>
<a name="wp81190"> </a><p class="pBody">
The <code class="cCode">SELECT</code> clause may contain an aggregate function with the following syntax:
</p>
<div class="pPreformattedRelative"><pre class="pPreformattedRelative">
aggregate_select_expression ::=
    {<code class="cCodeBold">AVG</code> |<code class="cCodeBold">MAX</code> |<code class="cCodeBold">MIN</code> |<code class="cCodeBold">SUM</code> |<code class="cCodeBold">COUNT</code> }( [<code class="cCodeBold">DISTINCT</code> ]
    cmp_path_expression) |
    <code class="cCodeBold">COUNT</code> ( [<code class="cCode">DISTINCT</code> ] identification_variable |
    single_valued_cmr_path_expression)<a name="wp81442"> </a>
</pre></div>
<a name="wp81189"> </a><p class="pBody">
Except for the <code class="cCode">COUNT</code> function, the path expression argument for an aggregate function must terminate in a persistent field. For the <code class="cCode">COUNT</code> function, the path expression argument may terminate in a persistent field, a relationship field, or an identification variable.
</p>
<a name="wp81459"> </a><p class="pBody">
The arguments of the <code class="cCode">SUM</code> and <code class="cCode">AVG</code> functions must be numeric. The arguments of the <code class="cCode">MAX</code> and <code class="cCode">MIN</code> functions must be orderable: numeric, string, character, or date.
</p>
<a name="wp81482"> </a><p class="pBody">
If the argument is empty, the <code class="cCode">COUNT</code> function returns 0 and the other aggregate functions return <code class="cCode">NULL</code>.
</p>
<a name="wp81476"> </a><p class="pBody">
If the <code class="cCode">DISTINCT</code> keyword is specified, duplicate values are eliminated before the aggregate function is applied. <cod