下面的查询应该为每个Reading_Type返回一行，加上该Reading_Type和date的已保存读取值，如果没有读取已保存，则返回0。
SELECT
t.*
, ISNULL(r.Reading, 0) AS Reading
FROM
Reading_Type t
LEFT JOIN
Reading r ON t.Reading_Type_ID = r.Reading_Type_ID
WHERE
r.Reading_Date = @date
OR r.Reading_Date IS NULL
Reading_Type表：
Reading_Type_ID Reading_Type
-----------------------------
1 Red
2 Blue
3 Green
Reading表（表格为空）：
Reading_ID Reading_Type_ID Reading Reading_Date
-----------------------------------------------------
@date = April 15, 2016的查询返回：
Reading_Type_ID Reading_Type Reading
----------------------------------------
1 Red 0
2 Blue 0
3 Green 0
Reading表（表格包含4月15日的数据）：
Reading_ID Reading_Type_ID Reading Reading_Date
-----------------------------------------------------
1 1 5 April 15, 2016
2 3 8 April 15, 2016
@date = April 15, 2016的查询返回：
Reading_Type_ID Reading_Type Reading
----------------------------------------
1 Red 5
2 Blue 0
3 Green 8
@date = April 7, 2016的查询返回：
Reading_Type_ID Reading_Type Reading
----------------------------------------
1 Red 0
3 Green 0
第三个查询仍应为Reading_Type = Blue返回一行，其中读取为0。如何修复查询？
答案 0 :(得分：4)
您的WHERE标准导致了您的过滤器问题（我自己只做了一百万次左右）。试试这个：
SELECT
t.*
, ISNULL(r.Reading, 0) AS Reading
FROM
Reading_Type t
LEFT JOIN
Reading r ON t.Reading_Type_ID = r.Reading_Type_ID
AND r.Reading_Date = @date
在此实例中省略WHERE子句（除非您想进一步过滤数据）。
以下是一些有助于详细说明此SQL功能的信息：Specifying Joins in FROM or WHERE clauses
答案 1 :(得分：1)
如果r.Reading_Date可以为null，并且您想要包含那些
SELECT t.*, ISNULL(r.Reading, 0) AS Reading
FROM Reading_Type t
LEFT JOIN Reading r
ON r.Reading_Type_ID = t.Reading_Type_ID
AND isnull(r.Reading_Date, @date) = @date
答案 2 :(得分：-1)
查询正在执行您要求的操作。它正在进行左连接（将返回r.Reading_Date中的所有记录以及来自NULL的记录（如果/当存在时）...然后它将应用您的where条件。如果@date既不是SELECT
t.*, ISNULL(r.Reading, 0) AS Reading
FROM
Reading_Type t
LEFT JOIN (
SELECT Reading_Type_ID, Reading
FROM Reading
WHERE Reading_Date = @date
OR Reading_Date IS NULL
) r
ON t.Reading_Type_ID = r.Reading_Type_ID
也不是<div id = "myCarousel" class = "carousel slide" data-interval="false">
<!-- Carousel indicators -->
<ol class = "carousel-indicators">
<li data-target = "#myCarousel" data-slide-to = "0" class = "active"></li>
<li data-target = "#myCarousel" data-slide-to = "1"></li>
<li data-target = "#myCarousel" data-slide-to = "2"></li>
<li data-target = "#myCarousel" data-slide-to = "3"></li>
<li data-target = "#myCarousel" data-slide-to = "4"></li>
<li data-target = "#myCarousel" data-slide-to = "5"></li>
<li data-target = "#myCarousel" data-slide-to = "6"></li>
<li data-target = "#myCarousel" data-slide-to = "7"></li>
<li data-target = "#myCarousel" data-slide-to = "8"></li>
<li data-target = "#myCarousel" data-slide-to = "9"></li>
<li data-target = "#myCarousel" data-slide-to = "10"></li>
<li data-target = "#myCarousel" data-slide-to = "11"></li>
<li data-target = "#myCarousel" data-slide-to = "12"></li>
<li data-target = "#myCarousel" data-slide-to = "13"></li>
<li data-target = "#myCarousel" data-slide-to = "14"></li>
<li data-target = "#myCarousel" data-slide-to = "15"></li>
<li data-target = "#myCarousel" data-slide-to = "16"></li>
<li data-target = "#myCarousel" data-slide-to = "17"></li>
<li data-target = "#myCarousel" data-slide-to = "18"></li>
<li data-target = "#myCarousel" data-slide-to = "19"></li>
<li data-target = "#myCarousel" data-slide-to = "20"></li>
</ol>
<!-- Carousel items -->
<div class = "carousel-inner">
<div class = "item active">
<img src = "includes/modules/sliders/Business_planning_for_success/Slide1.JPG" alt = "slide">
</div>
<div class = "item">
<img src = "includes/modules/sliders/Business_planning_for_success/Slide2.JPG" alt = "slide">
</div>
<div class = "item">
<img src = "includes/modules/sliders/Business_planning_for_success/Slide3.JPG" alt = "slide">
</div>
<div class = "item">
<img src = "includes/modules/sliders/Business_planning_for_success/Slide4.JPG" alt = "slide">
</div>
<div class = "item">
<img src = "includes/modules/sliders/Business_planning_for_success/Slide5.JPG" alt = "slide">
</div>
<div class = "item">
<img src = "includes/modules/sliders/Business_planning_for_success/Slide6.JPG" alt = "slide">
</div>
<div class = "item">
<img src = "includes/modules/sliders/Business_planning_for_success/Slide7.JPG" alt = "slide">
</div>
<div class = "item">
<img src = "includes/modules/sliders/Business_planning_for_success/Slide8.JPG" alt = "slide">
</div>
<div class = "item">
<img src = "includes/modules/sliders/Business_planning_for_success/Slide9.JPG" alt = "slide">
</div>
<div class = "item">
<img src = "includes/modules/sliders/Business_planning_for_success/Slide10.JPG" alt = "slide">
</div>
<div class = "item">
<img src = "includes/modules/sliders/Business_planning_for_success/Slide11.JPG" alt = "slide">
</div>
<div class = "item">
<img src = "includes/modules/sliders/Business_planning_for_success/Slide12.JPG" alt = "slide">
</div>
<div class = "item">
<img src = "includes/modules/sliders/Business_planning_for_success/Slide13.JPG" alt = "slide">
</div>
<div class = "item">
<img src = "includes/modules/sliders/Business_planning_for_success/Slide14.JPG" alt = "slide">
</div>
<div class = "item">
<img src = "includes/modules/sliders/Business_planning_for_success/Slide15.JPG" alt = "slide">
</div>
<div class = "item">
<img src = "includes/modules/sliders/Business_planning_for_success/Slide16.JPG" alt = "slide">
</div>
<div class = "item">
<img src = "includes/modules/sliders/Business_planning_for_success/Slide17.JPG" alt = "slide">
</div>
<!-- <div class = "item">
<img src = "includes/modules/sliders/Business_planning_for_success/Slide18.JPG" alt = "slide">
</div> -->
<!-- <div class = "item">
<img src = "includes/modules/sliders/Business_planning_for_success/Slide20.JPG" alt = "slide">
</div> -->
the "questions" slider starts here
<div class = "item">
<img src="../../images/top.jpg">
<div class="col-sm-4" style="color:black;">
<div class="">
<div><img src="../../images/Slide18.png" width="300" height="150"></div>
</div>
<strong>Scenario</strong>
<div style="color: #990000;">
<div><p>Siphelele's Sweets produces and sells fudge bars in Alexandra, Johannesburg.</p>
<ul>
<li>Each fudge bar is sold at R15</li>
<li>Her variable costs per fudge bar amount to R5</li>
<li>Siphelele's Sweets expects fixed costs of R30 000 for the year ending 31 Dec, 2016</li>
</ul>
</div>
</div>
</div>
<div class="col-sm-4">
<table width="734" style="margin-left:10px color:black;">
<form name="test1" action="" >
<tr><td height="30" width="578"><label>Questions</label></td></tr>
<div>
<tr><td height="30" width="578"><label>a.How many bars does Siphelele have to sell to break even?
</label>
<ul class="nostyle">
<li><input name="q2" type="radio" value="2000" /> <label>2000</label></li>
<li><input name="q2" type="radio" value="3000" /> <label>3000</label></li>
<li><input name="q2" type="radio" value="1500" /> <label>1500</label></li>
<li><input name="q2" type="radio" value="250" /> <label>250</label></li>
</ul>
</td></tr>
</div>
<div>
<tr><td height="30" width="578"><label>b.What is the break even point in sales given your first answer?
</label>
<ul class="nostyle">
<li><input name="q3" type="radio" value="30000" /> <label>30000</label></li>
<li><input name="q3" type="radio" value="3750" /> <label>3750</label></li>
<li><input name="q3" type="radio" value="45000" /> <label>45000</label></li>
<li><input name="q3" type="radio" value="22500" /> <label>22500</label></li>
</ul>
</td></tr>
</div>
<a class="btn" href="javascript:submitForm();">Check</a>
</form>
</table>
</div>
<img src="../../images/down.jpg">
</div>
，那么该查询中的记录将从结果集中排除。
我认为你想要的，如果我已经理解正确的话，就是在子选择中的左联接......所以更像是这样：
<script language="JavaScript">
function submitForm()
{
if ((document.test1.q2[0].checked == false)&&(document.test1.q2.value == "")){
alert ("Please answer question a");
return;
}
if (( document.test1.q2.value != "3000")){
alert ("Oops incorrect answer , please try question a again");
return;
}
if ((document.test1.q3[0].checked == false)&&(document.test1.q3.value == "")){
alert ("Please answer question b");
return;
}
if ((document.test1.q3.value != "45000")){
alert ("Oops incorrect answer , please try question b again");
return;
}
else{
$("#myCarousel").carousel('next');
}
}
</script>
</div>
<!-- Carousel nav -->
<a class = "carousel-control left" href = "#myCarousel" data-slide = "prev">‹</a>
<a class = "carousel-control right" href = "#myCarousel" data-slide = "next">›</a>
<!-- Controls buttons -->
<div style = "text-align:center;">
<input type = "button" class = "btn-sm prev-slide" onclick="slide()" value = "Previous Slide">
<input type = "button" class = "btn-sm next-slide" onclick="slide()" value = "Next Slide">
<script type="text/javascript">
function correct_answer() {
// Cycles to the next item
$(".next-slide").click(function() {
$("#myCarousel").carousel('next');
});
}
function slide() {
// Cycles to the previous item
$(".prev-slide").click(function() {
$("#myCarousel").carousel('prev');
});
// Cycles to the next item
$(".next-slide").click(function() {
$("#myCarousel").carousel('next');
});
};
</script>