June 11, 2011, 12:36 p.m.
posted by pythonics
Advanced Table Tags
While it is possible to build a simple table quickly, complex tables with varying border styles, running headers and footers, and column-based layout were not easily constructed from the old HTML 3.2 table model. Microsoft rectified this inadequacy somewhat by adding a number of table-layout controls to Internet Explorer version 3.0. These very useful extensions found their way into the HTML 4 standard and subsequently into XHTML. They provide row-based grouping and running headers and footers, along with column-based layout features.
Defining Table Sections
Within tables, all rows are created equal. In real tables, some rows are more equal than others. And most tables have header and footer rows that repeat from page to page. In large tables, adjacent rows are grouped and delineated with different rule lines to make the tables easier to read and understand. HTML 4 and XHTML support all of these features with the <thead>, <tfoot>, and <tbody> tags.
The <thead> Tag
Use the <thead> tag to define a set of table header rows. The <thead> tag may appear only once per table and is placed at the beginning, just after the <table> tag. Within the <thead> tag, you may place one or more <tr> tags, defining the rows within the table header. If given the opportunity, the HTML 4/XHTML-compliant browser replicates these heading rows when the table is printed or displayed in multiple sections. Thereafter, it repeats these headings on each printed page if the table appears on more than one page.
The ending </thead> tag is optional for HTML. Because the <thead> tag appears only in tables where, presumably, other rows are designated as the table body or footer, browsers automatically close the <thead> tag when they encounter a <tbody> or <tfoot> tag or when the table ends.
The many attributes of the <thead> tag operate identically, take the same values, and affect all the enclosed <tr> contents as though you had specified them individually for each <tr> enTRy. For example, the align attribute accepts values of left, right, or center, controlling the horizontal alignment of text in all of the heading's rows. Similarly, the valign attribute accepts values of top, middle, baseline (Internet Explorer only), or bottom, dictating the vertical alignment of text in all of the heading rows.
If you don't specify any alignments or styles, the browser centers the heading text vertically and horizontally within the respective cells, equivalent to specifying align=center and valign=middle for each. Of course, individual row and cell or stylesheet specifications may override these attributes.
The <tfoot> Tag
Use the <tfoot> tag to define a footer for a table. The <tfoot> tag may appear only once, just before the <tbody> tag. Like <thead>, it may contain one or more <tr> tags that let you define those rows that the currently popular browsers use as the table footer. Thereafter, the browser repeats these rows if the table is broken across multiple physical or virtual pages. Most often, the browser repeats the table footer at the bottom of each portion of a table printed on multiple pages.
The closing </tfoot> tag is optional in HTML because the footer ends at the following <tbody> tag or at the end of the table.
The <tbody> Tag
Use the <tbody> tag to divide your table into discrete sections. The <tbody> tag collects one or more rows into a group within a table. It is perfectly acceptable to have no <tbody> tags within a table, although where you might include one, you probably will have two or more <tbody> tags within a table. So identified, you can give each <tbody> group different rule line sizes above and below the section. Within a <tbody> tag, only table rows may be defined using the <tr> tag. And, by definition, a <tbody> section of a table stands alone. For example, you may not span from one <tbody> into another.
The closing </tbody> tag is optional in HTML because the section ends at the next <tbody> tag, or when the table ends. Like <tfoot>, there are many attributes for the <tbody> tag, but none is supported by the popular browsers. If you have special alignment attributes for this section, you'll need to specify them for each row within the <tbody> tag.
Using Table Sections
From a presentation standpoint, the most important thing you can do with the <thead>, <tfoot>, and <tbody> tags is divide your table into logical sections that are delimited by different borders. By default, Internet Explorer does not do anything special with the borders around the headers, footers, and sections within your table. By adding the rules attribute to the <table> tag, however, you can draw thicker rule lines between your <thead>, one or more <tbody>, and <tfoot> table sections, helping readers better understand your table's organization. [The align attribute (deprecated), 10.2.1.1]
For example, here is the simple table you saw earlier in this chapter, augmented with a header and footer. Notice that we've omitted many of the closing tags for brevity and readability of the HTML but that the tags must appear in an XHTML-compliant document:
<table border cellspacing=0 cellpadding=5 rules=groups> <caption align=bottom>Kumquat versus a poked eye, by gender</caption> <thead> <tr> <td colspan=2 rowspan=2> <th colspan=2 align=center>Preference </tr> <tr> <th>Eating Kumquats <th>Poke In The Eye </tr> </thead> <tfoot> <tr> <td colspan=4 align=center> Note: eye pokes did not result in permanent injury </tr> </tfoot> <tbody> <tr align=center> <th rowspan=2>Gender <th>Male <td>73% <td>27% </tr> <tr align=center> <th>Female <td>16% <td>84% </tr> </tbody> </table>
The table as rendered by Opera is shown in Figure. Notice that the rules after the table header and before the footer are thinner than the borders around the other table rows. This happened because we included the special rules=groups attribute to the <table> tag. You may obtain similar effects by specifying rules=rows or rules=all.
Use HTML 4/XHTML table tags to specially section your tables
Long tables often benefit from thicker rules every few rows, making it easier to read the tables. Do this by grouping the rules in your table with several <tbody> tags. Each set of rows contained in a single <tbody> tag will have thicker rules before and after it.
Here is an expanded version of our HTML table example, with additional sections set off as separate groups:
<table border cellspacing=0 cellpadding=5 rules=groups> <caption align=bottom>Kumquat versus a poked eye, by gender</caption> <thead> <tr> <td colspan=2 rowspan=2> <th colspan=2 align=center>Preference <tr> <th>Eating Kumquats <th>Poke In The Eye <tfoot> <tr> <td colspan=4 align=center> Note: eye pokes did not result in permanent injury <tbody> <tr align=center> <th rowspan=4>Gender <th>Males under 18 <td>94% <td>6% <tr align=center> <th>Males over 18 <td>73% <td>27% <tbody> <tr align=center> <th>Females under 18 <td>34% <td>66% <tr align=center> <th>Females over 18 <td>16% <td>84% </table>
The result is shown in Figure. Notice the Gender column? Netscape versions 4 and earlier placed it to the left and centered between the Males and Females rows, as you might expect. However, the HTML 4 and XHTML standards explicitly disallow spanning <tbody> sections so that the compliant browsers all display the example with just up to four rows in the table, separated into two groups. You could create any number of groups within the table by adding more <tbody> tags.
Multiple <tbody> segments further divide a table, but you cannot span across them
Defining Column Groups
The basic table model is row centric. Sometimes, though, it is easier to deal with your table as a collection of columns. Using the <colgroup> and <col> tags, HTML 4 and XHTML, as originally implemented by Internet Explorer through table extensions, help you turn the tables and think in columns.
Unlike the sectioning tags described in the previous sections, which are interspersed with the rows of a table to define headers, footers, and sections within the table, the column-related tags cannot be intermingled with the content of a table. You must place them at the very beginning of a table, before the content. They define the model by which HTML 4/XHTML-compliant browsers render the columns.
The <colgroup> Tag
The <colgroup> tag defines a column group. You can use the <colgroup> tag in two ways: as a single definition of several identical columns, or as a container for several dissimilar columns. You can put one or more <colgroup> tags within a <table> tag. The ending </colgroup> tag is rarely used in HTML but is required in XHTML. In HTML, the <colgroup> ends at the next <colgroup>, <thead>, <tbody>, <tfoot>, or <tr> tag.
All the currently popular browsers support <colgroup> and its attributes.
The span attribute
Use the span attribute with the <colgroup> tag to achieve the first type of column grouping. The value of the span attribute is the integer number of columns affected by the <colgroup> tag. For example, a table with six columnsfour in the first group and two in the otherwould appear in the source code as:
<colgroup span="4"> <colgroup span="2">
When the HTML 4/XHTML-compliant browser collects the table cells into columns by the example definition, it groups the first four cells in each row as the first column group and the next two cells into a second column group. Any other attributes of the individual <colgroup> tags then are applied to the columns contained within that group.
When to span and col
To use the <colgroup> tag as a container for dissimilar columns, leave out the span attribute, but include within each <colgroup> tag an individual <col> tag for each column within the group. For instance, in HTML:
<colgroup> <col> <col> <col> <col> <colgroup> <col> <col>
This method creates the same number of columns in each group as we had with the span attribute, but it lets you specify column attributes individually. You can still supply attributes for all the columns via the <colgroup> tag, but the attributes in the <col> tags will override them, as appropriate.
For instance, suppose we want our first example group of four columns to each occupy 20 percent of the table, and the remaining two columns to each take up 10 percent of the total table width. That's easy with the span attribute:
<colgroup span=4 width="20%"> <colgroup span=2 width="10%">
You can also create this structure with individually specified columns (in HTML):
<colgroup width="20%"> <col> <col> <col> <col> <colgroup width="10%"> <col> <col>
You can use both methods in the same table. For instance, we could specify our example column groupings, complete with width attributes:
<colgroup span=4 width="20%" align=right> <colgroup width="10%"> <col align=left> <col align=right>
Notice that this lets us align the contents of the two columns of the second group individually (the default alignment is centered).
The other <colgroup> attributes
The many attributes common to tables control the familiar aspects of each column in the <colgroup>-encapsulated column group. These attributes accept the same values and behave exactly like the equivalent attributes for the <td> tag.
The <col> tag
Use the <col> tag to control the appearance of one or more columns within a column group.
The <col> tag may appear only within a <colgroup> tag within a table. It has no content and thus has no ending tag in HTML. Use </col> or a lone forward slash at the end of the tag (<col />) for the required XHTML end tag. The <col> tag represents one or more columns within a <colgroup> to which an HTML 4/XHTML-compliant browser applies the <col> tag's attributes.
All the currently popular browsers support the <col> tag and its attributes.
The span attribute
The span attribute for the <col> tag, like for the <colgroup> tag, lets you specify how many successive columns are affected by this <col> tag. By default, only one is affected. For example, let's create a <colgroup> that has five columns. We align the first and last columns to the left and right, respectively, and center the middle three:
<colgroup> <col align=left> <col align=center span=3> <col align=right>
You should use the <col> tag only within <colgroup> tags that do not themselves use the span attribute. Otherwise, the HTML 4/XHTML-compliant browsers ignore the individual <col> tags and their attributes.
The other <col> attributes
The many attributes common to tables control the familiar aspects of the column defined by the <col> tag. These attributes accept the same values and behave exactly like the equivalent attributes for the <td> tag.
Using Column Groups
Column groups are easier to use than they first appear. Think of them as a template for how to format your table columns. Their main purpose is to create groups that can be separated by thicker rules within your table and to streamline the process of applying formatting attributes to all the cells in one or more columns.
Returning to our original table example, we can place a thicker rule between the column labels and the data cells by placing the column labels in one column group and the data cells in another (in HTML):
<table border= cellspacing=0 cellpadding=5 rules=groups> <caption align=bottom>Kumquat versus a poked eye, by gender</caption> <colgroup span=2> <colgroup span=2> <thead> <tr> <td colspan=2 rowspan=2> <th colspan=2 align=center>Preference <tr> <th>Eating Kumquats <th>Poke In The Eye <tbody> <tr align=center> <th rowspan=4>Gender <th>Males under 18 <td>94% <td>6% <tr align=center> <th>Males over 18 <td>73% <td>27% <tr align=center> <th>Females under 18 <td>34%</td> <td>66%</td> <tr align=center> <th>Females over 18 <td>16% <td>84% <tfoot> <tr> <td colspan=4 align=center> Note: eye pokes did not result in permanent injury </table>
The results are shown in Figure. All we added were the two <colgroup> tags; the additional borders were drawn by the rules=groups attribute in the <table> tag. For borders between column groups to be drawn, the rules attribute must be set to groups, cols, or all.
Example demonstrating the various HTML 4/XHTML table features